Is there any “realistic” reason why a mage couldn’t wear simple armor? Is armor really that heavy? Or is just some stuff leftover from DnD
This isn’t even a thing in D&D anymore, so let’s talk about where this is coming from and why it really doesn’t matter, unless you want it to.
It’s important to understand that the magic in the worlds you create isn’t real. You can base your magical theories off of real world mysticism if you want, but that will lead you more into a discussion in metaphysics, rather than, “can I cast spells while using a tower shield?”
D&D’s had a long history, and when it comes to Wizards (and Sorcerers) a lot of it trends back to one fictional character: Gandalf. Gandalf wandered around in robes, with a walking stick, so Wizards got the ability to wield a staff and couldn’t wear armor. The big floppy hat was optional, but encouraged. (There is a lot more to unpack with Gandalf, but this is, almost certainly, where D&D was drawing inspiration from.)
With the caveat that I never played AD&D in tabletop, my understanding with that edition was that Wizards were flat out blocked from casting spells while wearing armor.
My introduction was in 3rd Edition. This edition had a few major changes, including the ability to take armor proficiency, “out of class.” This meant it was suddenly feasible for players to roll up a human Wizard and immediately give them proficiency with light armor. The result was a rule called, “arcane spell failure.”
In D&D, spell casting has five distinct components: verbal, somatic, material, focus, or divine focus. Most spells only have a couple, and (as far as I know, none have both focus versions.)
Verbal components require the spell caster speak as part of the spell. Material components are consumed in the casting (there’s also a version that consumes XP off the character), Focus components are items which are needed for the spell but not consumed. Divine Focus components are the easier to grasp example here, because they’re usually holy symbols needed to cast the spell. Non-divine Focus items work the same way, just they’re not a holy symbol.
Somatic components are how the designers justified, “arcane spell failure.” The term “somatic” just means, “of the body.” In D&D terms it refers to very fine hand movements necessary for spellcasting. If a wizard wants to cast a spell with a somatic component, there’s a specific hand gesture associated with it, and they must replicate it perfectly. This is sensitive enough that wearing any armor could slightly “throw off,” the Wizard and cause the spell to fizzle. (The chance for this to happen increases based on the armor worn.)
So, this is where D&D is very D&D. Arcane spell failure applies only to “arcane” spells, and doesn’t affect divine spells at all. On the surface this sounds fine. Except, some spells appear on both Arcane and Divine spell lists. There’s no explanation why a Wizard needs to be much more precise when casting Bear’s Endurance, while a Cleric can cast it in heavy armor.
Starting with 4E, arcane spell failure has been mostly scrubbed from the game. From what I remember, 4E doesn’t even address Arcane Spell Failure at all, and I’m actually struggling to remember what armor proficiency even does in that edition.
From what I know, in 5e, proficiencies allows you to spell cast in that armor weight. (Though, I’ll admit, I haven’t read any of the 5e core books.)
So, that’s D&D. Other fantasy settings have their own rules, and they run the gamut. So, why did D&D go the route it did?
Probably balance. Originally the idea was that armor would make you considerably harder to hit in combat (conceptualization for how D&D’s armor system works is a discussion on its own), so, because spell casters would become godlike beings, it made sense that you’d want to keep them from also being effective melee fighters. Combined with there being some particularly nasty melee based spells in D&D which are (or were) high risk/high reward decisions.
Over time, D&D’s defense options homogenized considerably, and by late 3.5, (and also in Pathfinder) you could get a pretty respectable Armor Class on your starting character regardless of class. A Fighter or Cleric would be getting it from their armor, a Rogue would be getting it Dex bonus and their (lighter) armor, and a Wizard (or Sorcerer) would be getting it from their spells.
Modern D&D’s approach is actually pretty reasonable, it’s not that mages can’t cast while wearing armor, it’s simply that they spent their time learning their spells, and not martial combat. As a result, they never learned to fight in armor, and simply don’t know how.
This might sound a little ridiculous when you’re talking about leather armor, but it actually makes some sense for chain or plate combat, as both require training and conditioning, which an academic would (likely) lack.
Beyond that, in game systems, you usually see four approaches to mages in armor: Complete prohibition, negative reinforcement, positive reinforcement, and agnostic systems.
Complete prohibition is where the mage simply can’t use armor (or can’t cast while wearing it.) This usually isn’t a result of the armor’s weight, but may be other factors, such as the armor (somehow) cutting the wearer off from “the flow of magic,” or creating some kind of magical interference.
Negative reinforcement is where the mage is penalized for wearing armor, but can still use it if they’re willing to make that sacrifice. Technically, 3e D&D was an example of this, because the Wizard would need to take armor proficiency feats, and then still risk losing any spell they cast, though it was technically possible.
Positive reinforcement is where the mage gains benefits from going unarmored. From a world building perspective, this can be very similar to the previous group, however if your mages have specialized garb that enhances their magical powers, that would be an example of this. If said garb can also appear as armor, then we have the next example.
Agnostic systems are where the game (or setting) doesn’t care what the mage is wearing. They can wear any armor they like without affecting their ability to use magic (though they still may need training to use the armor effectively, and as a result may abstain from armor.) As mentioned in the last paragraph, if you have specialized magic enhancing garb which also appears in armored variants (and that’s the only difference) then you’re probably looking at this. D&D’s Clerics are (ironically) an example of this.
So, you may have noticed I dropped D&D into two different categories here, and that is something to keep in mind for a sufficiently diverse world: Not every magic user is going to be following the same rules.
D&D’s Clerics have no restriction when casting in armor, their Wizards have to be very careful about armor because it impedes their hand gestures, and Druids have sworn oaths against wearing metal armor (if broken, these oaths suppress their their spellcasting.)
The structure of D&D encourages creating characters within limitations. This can be a very good thing from a character building (and world building) perspective. When any character can do whatever they want, they will tend to blend together. If your mage doesn’t wear armor because they’ve never trained to use it, and they can use their magical prowess to defend against attacks, that will give them a very different identity from a walking slab of meat wrapped in steel.
Further, when characters are limited by what they haven’t learned (or cannot do) because of their background, that will encourage a more diverse world, with a larger number of distinct groups and factions.
There’s also a potential for very direct explanations, like saying that a mage in your world shouldn’t cast electrical spells while wearing steel, because it will arc back onto them… but lightning’s fine, because that’s a bolt of plasma.
Similarly, geas are another potential restriction. Your character has magic, but it’s restricted by specific taboos. D&D has a bunch of these including the Druids (mentioned above), Clerics, and Paladins. If your setting has nature mages who lose their abilities if they, “use the artifice of civilization,” that could be a very chunky (or even debilitating) restriction.
Because you’re creating your world, you have a lot of freedom to say why magic does, or does not, play well with armor. D&D used it as a balancing mechanic, but turned it into a world building element until later abandoning it. While I’d strongly recommend staying consistent (you’re not cranking out multiple editions of a 45 year old RPG, and adjusting it to keep it fresh, and balanced-ish), it is a decent reference point for considering options in building your own fantasy setting, just don’t be afraid to step well away from it if you’d prefer to do something else.