Or the fact that they don't like you back...
Or, if you go out with them, shit will happen and you won't be close friends with them anymore. Not everything is about appearances.
I have a friend who is Mormon. She holds several beliefs (because of her religion) that I strongly disagree with. I can be friends with her, but I don't think I could date someone who believes some of the things that she does. Also neither of us is a lesbian.
I wouldn't date one of my good friends because he's a dumbass and we fight too much, but he's incredibly good looking.
I wouldn't date one of my best friends because he lives too far away and he does a lot of drugs (I've dated druggies before, they are typically horrible boyfriends). But he's attractive too.
So it's not all about looks.