They’ve been seriously attempted. DARPA’s been funding research into less than lethal weapons for decades. I’ve seen video of prototype net guns and glue guns that first got released in the 90s. I assume there were serious issues with both, as neither moved into production since then.
Tazer shogtun shells are commercially available, but they’re expensive, and won’t work against someone wearing heavy clothing.
Of course Tazers and stunguns are a real option for putting someone on the ground. Zap someone with a heart condition and you’ll probably outright kill them, but otherwise, unless you don’t keep zapping them for over thirty minutes, you should be able to handcuff them.
DARPA’s other dream is sonic weapons. Initially they were looking at battlefield application, but it sounds like what they had started trending more for riot control.
Anyway, shotguns have a lot of exotic shell loadouts designed to not kill the victim. Beanbag rounds are a standard as are rubber bullets. There’s also grenade variants.
Beanbags distribute the force and will usually put someone on the ground at medium range with acceptable risk. Fatalities from these things run around 2%. The also fail to incapacitate the target with a single shot most of the time. Which can actually lead to needing to kill the target, because, we’ll, now you’ve shot at them.
Rubber bullets or riot slugs are in basically the same category. These are low powder load shotgun shells with either plastic or fabric slugs or shot. They’re used for riot control, and can still leave some nasty injuries or outright kill their victims. As with beanbags one shot isn’t likely to be enough.
Gas shells come in a couple varieties. They can be tear or pepper gas, and spray a cone of the gas or launch a miniature grenade. Both grenade variants also exist as 40mm variants, and in handheld form. Incidentally, there’s a 12 gauge fragmentation grenade in development.
Bolo shells do exist, but they’re advertized as being capable of decapitating whatever you’re hunting… so, not exactly the definition of non-lethal I was looking for.
Moving away from shotguns, there’s a lot of grenade ordinance that ends up in the less than lethal category.
Concussion Grenades, or flashbangs, are designed to emit enough light and noise to disorient someone for a few seconds. It should be enough time to get handcuffs on someone. While actual deaths from flashbangs are fairly rare, they’re still a live grenade detonation, and they do generate enough heat to cause severe burns and start fires.
There’s also of course, pepper spray in all it’s variations. This stuff isn’t 100%, and can get into trouble when dealing with people who aren’t affected, or have been exposed to it previously. Unsurprisingly it can also cause a lethal reaction in some cases.
So, yes, less than lethal weapons do exist. None of them are 100%, and most of them can accidentally kill the target. The entire point is to have an option that will briefly incapacitate, without having to go directly to lethal force.