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I think most of the appeal comes from the fact that most people are fans of sports they have played before because they appreciate seeing the sport being played at its highest level of competition. Everyone has some sort of hobby or activity where they would be thoroughly entertained watch someone else do it at a much higher skill level than they can do it themselves. 
As for people who are fans of sports they have little experience with (i.e. me with football), there’s usually a little thing they like about a sport that gets them into it, like a certain player or playstyle of a team, and then once they get more context of why they like a certain player or playstyle through some research, they’re swept up with the sport.
Sorry for the rambling lol
Some people are just really into it. It’s a good small talk topic at work because most people have a sport they are into. I’m really into baseball, but I try my best to not rant about it to coworkers because not everyone likes baseball >

I have several friends that like baseball. I can totally understand why people to do talk about sports since uts so common, I just don't see the appeal personally lol.