Perhaps the OP was talking about sports teams in particular? Forming sports teams based on geographic area fits well, and gives people someone to root for without alienating other groups for what would amount to wrongfully discriminatory reasons in other contexts.
But if geographic region isn’t allowed, then finding other criteria can be hard. Who wants to root for the A’s to win against the B’s if there’s nothing besides the name that makes them different?
The EU is an organization that transcends countries and is vested with political power, and despite being constantly bashed by far-right parties in Europe, has built a single market in Europe and many new freedoms for it citizens, like the right to move to and work/study in other member states.
Nearly expelled Greece for not bailing out the German banks, but Hungary can compulsively retire judges, expel universities, and close newspapers without even a slap in the wrist.
Names and history are important. That's why there has been such a fight over renaming US teams with racist names.
The fans don't particularly care that it's owned by an american and sponsored by a UAE airline, because what matters is the coming together of the fans behind a common cause.
The fans do typically come from the area around the stadium in North London.
When you get just a little bit further down the league it's still very much a geographical thing because the teams are less well known and less successful, so are less likely to attract people who want to support "the best team". For example, Southampton FC fans are pretty much exclusively from Southampton.
