Because they are. They are the greatest single unifying force in our culture today, more than music, more than movies or television, more than anything, and especially among men. They bridge the gap between rich and poor, black and white, east and west, most anything. Frankly, that's the only reason any city would consider spending money on new sports stadiums. Because while the economic benefits are debatable, the social benefits are not.
Cities just have to decide how much they're worth.
I can remember being at Shea Stadium in 1969 on the night the Mets clinched the National League East Division, the first title ever for a hapless franchise. As the game ended, hundreds of fans stormed the field, many more than you usually saw at those kinds of things. They also tore up the turf, claiming souvenirs. My most vivid memory of that night is not from the game, but from the train ride home because the guy standing next to me was clutching this hunk of sod the whole time as if it were gold. He said he was going to plant it in his back yard.