I think it's because radical and possibly horrifying transformations (and also other weird things like inflation, or TG or whatever) were a part of a lot of western cartoons at the time a lot of today's 20-40 year old artists were kids/teens and it influenced their work.
Base +6, Tove +8, Megazord0h +6, somercet +8, Vasnody +2
Some people like domination, other are exicted by exhibition, some love reading hentai comics about giantess, or masturbation, you could prefer gay, lesbian, furry or anything. Transformation is just a preference among many.