they also don't want to make a bunch of different models (probably not the right term for what I mean but I know next to nothing about game development) for all the weapon and armor combinations
Rig. Attachment/accessory bone. You can't glue things directly to a skinned mesh (the model of a character that deforms for animation) because it'll be warped and your engine doesn't know exactly where that point will be at any given time (without doing some stupid shit that you shouldn't do, anyway. It is technically possible) so you need to include an extra invisible limb in the animation skeleton to indicate where the sword goes, basically.
But this is not a big deal so floating weapons are basically just laziness (or an oversight where you never told your artists that the characters could remove their cape, or whatever). It's not a big deal to include a second attachment point for when they're wearing less, or blend another "animation" that just articulates the attachment "limb" to the correct offset, for example.
In some cases maybe they don't bother because it's kind of a fudge in the first place (like, how often do they actually have a sling/scabbard there? Practically never, everyone just has a magnetic back) and there's some ugly clipping going on anyway so they don't want to think about it and they don't want you to think about it. Especially if there's a literal cape involved, or if the sword goes on top of a shield and the shield might be in-hand while the sword is still holstered so the offset would need to magically warp around which doesn't make sense either. But yeah it annoys the fuck out of me too.
Edit: oh yeah, there's also the fact that if you change the offset of the slung weapon, the character's hand won't be in the right place when they reach back to grab it. So that actually does require either a second animation or some minor IK or something, which is less trivial.
...However I'd argue that (a) that shit usually happens so fast that it wouldn't be noticable anyway, and (b) if you're going to ignore it and have jank anyway, that would be a FAR BETTER option since it's an occasional misalignment for a frame or three versus something that's staring you in the face as you run around for hours.