Everyone's got their reasons. There's no way to get a real, definitive answer to this question. I've made a post about ONE reason before, where people play as the gender they feel comfortable being (e.g: transgender people). Then there are people who say "lol i like staring at a girls ass" in a game where you barely spend any time in a third person perspective; the list goes on and on. It's not stupid, it's just a thing people do.