Two reasons.
One, the system of it is done like soccer, where the people who go on to the finals aren't decided by the number of wins but the cumulative number of goals.
And two, so that there's a position that's infinitely more important than the rest that they can put the protagonist at.
Though the thing about Harry Potter that I noticed is that it's really two separate stories. There's the first four, which are lighthearted tellings of school, but with magic involved. And then there are the last three, which are dark teenage stories about rebellion against society, racism, murder, and EVERYONE DIES IT'S SO SAD!
Harry Potter books aren't well written. They're fun, but they're really pretty basic.