Student government, much like homecoming/prom royalty, is nothing more than a school-sponsored popularity contest. Most high school students realize that their student government is that in name only, having no real authority. No matter who they vote into these artificial positions, it will still be the teachers and faculty that run the school.

So, the kids do what they would have done anyway, and vote for their friends. Naturally, the "popular people" win, having more friends who vote for them.

Why do schools do this? By helping to emphasize who is popular, they are only doing harm to the less popular people.