it's entirely a social construct and I see no reason why men aren't expected to shave their legs either. (hint: I do. It's called sexism)
Personally I'm not fond of underarm hair, but leg hair I couldn't care less about unless it's really dense and dark. This is men and women I'm talking about, mind. Plus if I'm running my hand up someone's leg I'd rather feel lots of long hairs than a few short and blunt ones. Those are scratchy, man. No matter what someone's hair is like though, they can do whatever they like with it and it's not up to me, so.
That said I do shave my legs every so often. I tend not to be arsed shaving unless I'm going to be wearing bare legs, though.