Why Do Guys Only See Me as a Friend?

Ok, so I'm almost eighteen and all my life guys have only seen me as a friend. Up until a couple years ago everyone thought that I was a lesbian. Lila the lesbian was one of the many nicknames I had. I'd also worn glasses up until the last few months of high school when I finally got contacts. Suddenly more people started to talk to me and I made more friends, but that's all guys saw me as. I was the cool girl that all the guys could hang out with, but I've still never been asked out. At the same time, my younger brother is really popular and so I think that maybe that's why more people have been talking to me. I also think that maybe guys are too afraid to ask me out because my brother is really big and muscular and their afraid of him. I consider myself to be slightly above average when it comes to beauty, I'm more baby-faced and look like I'm fourteen and I'm not fat, but I'm not a stick either. I'm just wondering why guys don't see me as a girl. If there is something I can change I want to do it before I start college.