Because if there's a single creature that doesn't drop loot, then people will refuse to buy the game, the developer will go broke, and another game series will be dead.

You could also argue that it represents going to the animal's lair and finding the only useful bits left after it ate some random explorer. But that would be boring, so you don't have to play that bit out. But basically it's "players expect 'experience' money and items from every kill no matter what" and game developers give us what we keep buying.

At least Dragon Age had wolves drop wolf pelts as well. That made some sense, although I'm not sure that taking time out to skin a wolf during the middle of a battle was substantially more realistic.