"Why can't they just buy snake food?"
because snakes eat animals. That's what they are designed to do.
Good snake owners will pre-kill their prey.
but snakes do not do well on a "kibble" diet. They actually are only force fed a snake diet if they won't eat on their own and is done in order to save their lives. But most snakes will readily take a thawed or pre-killed prey. That's what they live on. They don't go around eating plants or other such things. Some snakes will eat eggs, but most won't.
Rabbits are a prey animal and hence one feeds prey animals to snakes. (rabbits, rats, mice, guinea pigs, small pigs etc) these are good food animals for snakes.