You do realize that you're saying all hot chicks have bad personalities and are stuck up b*tches. That is not true. Surely if you attempt to talk to an attractive chick she'll turn out to be nice (not all of em ARE nice but they do exist believe it or not).
Most Helpful Girl
Well, luckily, it is most likely because they really like the girl and find her attractive even though you may not.
I see you are under 18, so even very attractive guys in your age range are not so filled with high self-esteem as you assume. They may act quite confident and be confident in many areas of their life, but that does not mean they find it easy to approach a super attractive girl.
Beyond that, they may believe a very attractive girl will demand to be treated in a way they don't feel like dealing with. They may assume you are high maintenance, difficult, judgmental, and could dump them in a heartbeat for another guy if they don't measure up. Think of how "hot" girls are stereotyped in a lot of teen movies. They are portrayed as gossipy mean girls who expect guys to break out a lot of $$ to date them.
Oh, and regarding sex, the opposite can be true. Some guys will pursue a hot girl simply for sex. I had an ex who dated a girl he thought was amazingly hot, his perfect woman (physically). He dated her a few times, spending as little money as he could while still treating her the way she apparently expected to be treated, and then he got her into bed. He thought she was annoying and he had no desire to spend his hard earned money dating her, so he definitely had no desire to enter into a relationship with her and stopped seeing her quickly.