They generate TV ratings . . . It's not about the athleticism, sportsmanship, or anything else, it's about the TV audience. It shouldn't be thought of as a sport so much as an entertainment option. So many people watch the game, it allows the league to charge high advertisement rates, which generate cash to pay the actors / athletes. The average length of a career for an idiot who chooses to play in the NFL is slightly less than three years, and I recently saw a stat that around 70% of players declare bankruptcy within five years of leaving the league.
There are the obvious exceptions for star players, but playing in the league is a horrible deal for most players, considering the risks involved. It's a cash cow for the owners.