I also think the whole idea of fandom is different in the US too! Maybe because people move around more and pro sports teams are more spread out? I grew up in the Midwest and there were a few pro sports team within 4-5 hours. So people end up supporting different ones, even if they live in the same area. And kids today tend to follow players more than teams. Don’t know if that’s the case in UK?
I also think a lot of European supporters are attracted to the older sports franchises - Sox, Cubs, Yankees, etc. Probably more visibility. That’s partially why I started supporting City. Always on TV! But I like your comment about misery loving company - my husband supports the Spurs because it’s like 1990s Red Sox.
Article - If you like American sports you might find this article interesting - as stupid as it sounds, I know a lot of US football supporters that picked their team based on this article almost 20 years ago.