That's because typically a 4legged animal is more developed and capable of processing what's going on than an insect or worm, at least as far as we can understand.
I'd kill 100 rats to save a cute babies life, but that doesn't make it right, but that's what I'd do.
However to kill 100 rats, or in the case of corpse-munchers, kill 50-100 animals a year simply because they want a momentary sensation of flesh and the knowledge that it's "real meat" is not just immoral, but inexcusable. It's senseless. It's not to save another more valuable (subjective) organism, no, it's just depravity.