Why? Because we're FANS!!!
I live near a bunch of Steeler's Clones, they do it too; all fans everywhere do it. It's why the NFL is so popular, teams become part of a local identity. Go anywhere, say you're from Buffalo, and very soon the subject of the BIlls will come up.
Our teams become part of the DNA of our local culture. Why do small towns across America shut down on Friday nights? Because the local HS team is "our" team, the names may change each year, but they will always be our team.
By the way, take a drive out of the city or suburbia, find a relatively successful small town high school team, and go to a Friday night game (preferably the Homecoming game). It reminds you why you love the game, and this country.