Bit of an unusual question, why are halberds and other pole arms curiously absent from most fantasy?
I’m just going to start out by saying, there’s no single answer to this. When many writers are making similar decisions, you can sometimes track back to a singular source, but when you’re dealing with an entire motif like this, writers (and world builders) are making their decisions independently of one another.
The first possible reason is inspiration. When you’re creating your fantasy setting it’s very likely you’re drawing inspiration from somewhere. Many fantasy authors (intentionally or otherwise) draw inspiration from Lord of the Rings. This is so ubiquitous that you don’t even need to explain concepts like Elves, Orcs, and Dwarves to an uninitiated reader. It’s so ingrained in the cultural lexicon of fantasy that defining these things doesn’t even seem necessary.
Actually, a bit of fun trivia to think about with this: the correct terms in English are “dwarfish” and “dwarfs.” The terms “dwarven” and “dwarves” aren’t completely original to Tolkien, but in modern literature they trace directly back to him.
If we’re going to lay blame at the feet of Tolkien, then it’s worth remembering that his work does include polearms, however, those weren’t the weapons used by the heroes. Tolkien’s heroes were designed to be legendary leaders, and their weapon choices reflected that to an extent.
If you’re creating an adventuring hero who is secretly the lost heir to the throne, you’ll give them a sword as their weapon (unless you’re deliberately being subversive.) There’s nothing wrong with this, it’s a legitimate aspect of how western society looks at the sword as weapon, but it is worth remembering.
Now, if you’re drawing inspiration from a story about a character who’s a lost scion, or has a sword as their primary weapon for some reason, then it’s distinctly possible you’ll lift the weapon choice without thinking through why.
I’m less confidant that a similar logic applies with axes. I’m not completely certain what Tolkien was referencing when he picked the axe as the weapon of Dwarven kings, but here we are. If your setting is heavily influenced by cultural elements from Northern Europe, or he could have simply picked the weapon for more novel reasons.
In all of this, the spear, to say nothing of more advanced polearms, gets lost in the shuffle. This doesn’t mean that there aren’t fantasy settings that use polearms, but, like you said, they’re less common. In the long term, this creates a kind of “authorial confirmation bias,” where you’re more likely to be influenced by fantasy settings that eject or discount polearms, than you are to immediately think of these.
Another factor is, very often, a polearm isn’t a good weapon choice for the kind of character you’re writing. This is a little more subjective, because your world building could easily go either way. However, in general, the wandering adventurer is more likely to be in situations where they need a sword, axe, or dagger, far more often than ones where they’d need a spear.
There’s a slightly speculative quality to this logic, which then becomes self-confirming when you combine it with the previous element. If your character is a specific kind of roving adventurer or mercenary, and you expect that kind of a person to carry a sword or axe, then you’re more likely to give them a sword or axe. Again, historically, in Europe (and elsewhere) they’d probably carry a sword or axe as their sidearm, with a spear or other polearm as their primary weapon.
To be fair, there is some basis for this. If you have a character who’s a member of the city guard, or isn’t in active combat, they may not carry their primary weapon around with them everywhere, and might restrict themselves to their sidearm. This is somewhat analogous to a modern era character who carries a handgun, but they’re not going to wander around with a shotgun or assault rifle as part of their day to day gear. It’s likely something they’d have on hand, but wouldn’t carry regularly.
Also worth noting that in early modern Europe, it was fairly common for nobility, and other members of society to carry a sword as a normal accessory. Over time this fell out of fashion, but there is some basis for the idea of a character who carries a blade, instead of a polearm.
Ironically, greatswords and greataxes would fall into a similar situation. Those are primary weapons, not sidearms, but many fantasy settings will gleefully include them while ignoring polearms.
Another factor which may be relevant, though I’m not sure exactly how relevant is swashbuckler films and literature, and the resulting pulp genre. This included scenarios with combatants who would have eschewed polearms for various reasons. For example: Anyone who engaged in ship to ship combat, such as pirates, or naval officers hunting pirates. To be clear, I’m not blaming The Three Musketeers for squelching polearms in fantasy literature, but there is a progression from this material, through early fantasy pulps like Robert E. Howard’s Conan or Fritz Leiber’s Fafhrd and the Gray Mouser into modern fantasy.
Modern fantasy takes, pretty much, equal measure from the early sword and sorcery pulps, and blends that with Tolkein’s intricate worldbuilding. The result is, slightly idiosyncratic, but it does start to explain why a lot of authors might eschew spears or halbards when they can just give their characters a huge axe. It’s also part of where fantasy settings become anachronistic.
If you’ve never read it, Conan is deliberately anachronistic. Robert E. Howard loved history, and gleefully grabbed the bits that appealed to him, mashing them together with reckless abandon. The result is fantastic writing, but there is no way to reconcile the Hyborian Age with real world history. The names are the familiar, but everything else went into a blender.
Fafhrd and the Gray Mouser is a similar situation, though the names are alien in ways that should be immediately familiar to anyone who has read modern fantasy. In some ways, it’s even more significant, because the fictional city of Lankhmar was designed to be a fantasy allegory for 1930s New York, with completely inconsistent technological advancement scattered across its setting.
To be fair, I may be overthinking this. As I mentioned earlier, authors read one setting, it becomes a part of their understanding of the genre, or at least a familiar touchstone. When they go to create their own works, the things they’ve read (or viewed, played, consumed in whatever form) influence their work. So, while the specific examples I’m giving influenced a lot of writers, it’s not like those are the only possible paths. It’s also worth remembering that many authors will get their point of contact further down the line. So, they’re picking up on the influences of someone else’s influences.
This may sound like a nonsequitur, however, no one’s work is ever, truly, original in some cosmic sense. You’re influenced by everything you’ve read, watched, played, or otherwise engaged with. It becomes a part of you, and a part of how you look at the world. When the time comes to write, those influences will affect what you create. Being aware of this means you can step back and have the self awareness to start to deliberately change things.
If you want to see fantasy that uses polearms more heavily than what you see now, you’re certainly welcome to. There are plenty of reasons for your characters to use them. Especially in character archetypes that normally eschew polearms in conventional roles. For example, spears have been used in hunting for thousands of years, so it would make perfect sense for your ranger or druid to carry one as their primary weapon. Of course, most polearms saw use on the battlefield, and that’s certainly one use. You may have characters who are members of the city watch, or a similar group, who break out the polearms when things get dicey. This is before you consider the idea of arming characters like your clergy or mages with weapons you usually don’t associate with them. Again, there could be any number of reasons, you’re only limited by your creativity, and the ideas you found abandoned along the way.