Never in my life have I thought that while having sex, have you?
"Hmm I wish I could see my ass while banging this chick"
Likely not, for sure, but from the side, or some other angle, so you can see more of her, that is the trick.
Have you ever tried to pose a camera from first person? It is not easy, you have to do things like break the MC's neck or remove the head entirely, it's messy at best. Also, you might not be aware of this, but not everyone likes first person, so you have to have options for everyone, that is why this game, and many others give you options for that, so you can see some of it from first person, and some of it not.