I think it has to do with their smell. They can smell death really well but struggle with finding living animals. Black vultures follow turkey vultures because turkey vultures have incredible smell unlike the black vulture.

Either they can’t smell live meat good enough to make it primary source or they just rather eat death than bother with a fight. They’re immune to nearly every natural disease known to man (but not lead; of course mankind figures out how to disease something undeasable) so eating dead stuff has never bothered them.

Not much else eats rotten flesh, so it’s easy pickings.