if it's not difficult to implement
1. how come no developers ever do it? It can not be that all of them are lazy.
2. if it's actually easy, then there must be one exploitable trap behind that idea that everyone always drop it.
1. This is true enough. It probably comes down to other design issues. In most cases developers are the least to blame, after all they are taking orders from designers. Designers typically run their dev shops like Stalin or Hitler. No dissension in the ranks, etc.
2. Designers don't typically think like devs, nor do they typically think in-universe like writers. The player will kill enemies. The enemies will drop sweet loots. Repeat. Hey, look, I just made a vidya game! They don't wanna challenge you too much, regular people hate being challenged.
In all reality, it may be the nature of most games. Inhuman enemies and demons typically don't carry much worth picking up, so just give them a drop table. Trash mobs like wolves only have their skin (can't drop that), and so you're left relying on humanoids for mechanics like this. It's hard for most designers to think their way into this situation and still convince themselves its a good idea. I can't entirely blame them, I mean, they spent ages trying to contrive a way for players to murder a buncha dudes. Don't break the status quo. Keep it simple.
As for most indie devs and niche designers, well, Idk. I'm sure there are some obscure titles out there that do it, ones I'm just not aware of yet. Neo Scav does it. Hell, Neo Scav will let you kill a guy, then carve up his corpse, cook up the parts, and eat him while wearing his old trousers. Is this particularly brilliant?
Fuck yes it is!