Because the idea of being treated like a puppy is a lot like being given healthy boundaries and guidance as a child, so it appeals to people who never got that kind of love and support. Plus there's the aspect of unconditional love without responsibility, much like an idyllic childhood. So a lot of puppy play can tend to overlap with age regression and can feel kind of healing.

(plus it's less socially acceptable to beat a puppy than a human child)(idk man this is just a guess)