My working theory, and one I’d be happy to amend provided with the right evidence, is that the particular weird competitiveness that most team sports elicits in its fans (and even its players) is ultimately kind of destructive. I mean that seriously.
I think you sell this theory short! American politics is totally an us-vs.-them affair that owes a lot to sports fandom. (It would of course be uncharitable to compare these emotions to others governed by Godwin’s Law.)