You know, we could easily flip this question and ask (similarly) why do all the women go for the same guys and the answer would be the same.
There are certain qualities that are attractive to almost everybody. Whether its good looks, charming personalities, etc. No matter where you come from or who you are, there are certain baseline characteristics that tend to draw most people in. That's your answer in a nutshell: the girls that all the guys seem to be after are the desirable ones because of their looks for the most part. Hell, it seems like nearly every teen movie ever conceived was built around this premise, sometimes with the guys being the desired ones and sometimes with the girls being the desired ones.
So no, it's not a competition. Not in a traditional sense. It's social-darwinism. Survival of the fittest at work. People seek to mate and deal only with desirable individuals. Even when two folks who look like they have no business finding love find eachother its because they managed to find one another desirable in an evolutionary sense.
Quite simply, we all go after them because they are the ones who, for whatever reason, are desirable and make themselves wanted.
Wait what? Anonymous? Why isn't anyone taking credit for this answer?