There isn’t just one reason. The biggest one currently can be directly tied back to porn. It’s also not just the simple existence of porn, but with our excessive consumption of it.
The fact that nearly all porn is the same in this regard is also a factor. There’s also a generational factor, in that younger people experienced almost-exclusively shaved partners and porn in their formative years.
People of all ages have different levels of beauty, and not in a sexual way.
Ever thought a kid was cute/adorable/whatever? And the kid next to them was not?
On the complete other end, many elderly are visually repulsive, but some look good right to the end.