Girls your age tend to go for a guy based solely on his looks, therefore the ones that look the best are more likely to have been taken already. Wait a bit, and people start to gain their senses and become attracted to people based on a much more diverse set of factors. That's when people become single or attached because they "match" other people, not just because of their looks.
Not to be rude but you seem a little shallow. You should not look for just good looks. Look for personality and values. About the guys that have girlfriends. Well there are two things that may be happening...
1. Bad Luck
2. All of the girls are all over the good looking ones. It's only natural that the best looking males have a girlfriend.
Note: By the way we men have the same problem. It's hard to find a good looking, kind, fun, normal girl. (basically the package deal) Now we're not looking for perfection but there is usually something wrong with her or she's taken. Now by something wrong I mean like she lives too far away, she doesn't like you, she has a weird family, etc.
Have you heard of the line "all the good guys are taken" well that's true in most cases. Every girl wants a good looking guy with a good personality, girls just got to them before you did. You are young so of course you're going to meet plenty of guys and some of them will be single. Don't look for the perfect guy based on looks because looks won't get your relationship far in most cases.
Because a lot of those guys think they have to have a girlfriend to complete them and a lot of them get by with their looks so therefore they will always have a gf. But maybe it's not that you can't find single guys maybe it's just because you focus on their looks more than anything else. If you really try and get to know guys you'll realize it's more than just looks and a great guy could be around the corner you may not have found him yet.