What's your favorite body part on a guy? And would you like it more if every guy walked around in clothes that showed off that body part or if every guy walked around in clothes that completely hid it so that you had no clue what it looked like? I bet that I know what your answer is going to be.
Well, according to guys, legs are the sexiest part of a female body (in theory) so they obviously LOVE the fact that a girl wearing shorts or jeans would be a tease show. They find it attractive and sexy. Legs define a women and guys like that. Thats just the way guys are and its completely natural. Its like a dude asking "why do women love athletic men with shirts off?" ... well because its hot and a total turn on.
So in whole, it gives women the appealing character and guys love it.