Seems to me it's because so many people don't have coherent political ideals of their own and are just identifying with this or that player or team. Lack of media literacy and the corporate control of mass media play into propagating this problem further, though it starts with upbringing, much like a religion.

There are ways to debate political issues without tying anything back to political parties. For a long time now, we could have used more of that both in schools and in the media.