Why do football fans care about football? None of it affects them. If someone moves teams, and gets sold for a bazillion dollars, the fans don't see any of the money.
Why do petrolheads care about fast cars? They'll probably never drive a Lamborghini, so why bother reading about Lamborghinis?
Football is simply more popular than box office; that doesn't make it any more worthy of your attention. For a small number of people, box office is fascinating and fun.
Me personally, I'm interested in the film industry as a whole, and I've always liked statistics. Following box office is interesting to me, and comparing the trends, writing spreadsheets, seeing how accurate I can predict, is fun.