zoophilia is the attraction to animals, often times also related to the sexual abuse of animals. whereas petplay has nothing to do with real animals, its a sexual dynamic not an attraction, its roleplaying being in the role of a pet, its about fantasy and play, its a human in a pupmask, with a collar and tail, its neko girl shit. sometimes petplay can be non-sexual. sometimes its explicitly sexual. alot of petplayers get emotional fulfillment from it, comfort, safety, enjoyment. its kinda difficult to explain but, its far more about imagining yourself in a certain situation, that being a pet.

as for "why animals" think about golden retriever boyfriends, cat girls, ect. people just connect personalitys and vibes with certain animals, its normal to some extent. some people get really into this persona of being part animal. it gives people a sense of self outside of what is often a really boring existence. "im not just submissive and clingy, im a puppygirl, and i can play into that part of myself with play pretend and my owner loves and accepts me for it and i can take joy in aspects of myself i didnt know i could find hot previously."

the difference in drawn content is that its humans with animal features. it can cross over into furry territory but the common consensus for zoophilic art is feral & non-anthro animals,
(this was a really shitty explanation but that's the basics)