It is all very simple business. Why should Bob continue to take 50.00 of the 75.00 that Joe is making off of Bob's product when Bob can get himself set up to take the whole 75.00? And it wouldn't be some new innovative business plan, either. Cutting out the middleman has been a tactic for improving profit almost since middlemen sprung up.
I think you're making a HUGE assumption that the income would be "75" wherever they go.
Part of the "75" is due to the NFL broadcasting on major networks in the first place. Viewership would drop off if they moved away from network TV.
CBS, NBC, ABC, ESPN, FOX have a lot of viewers who tune in for other programming than the NFL. Cross promotion is a big key to drawing an audience. Someone watches CSI, sees a promo for the next big game and decides the watch it.
I alluded to boxing. "Out of sight, out of mind." Once boxing broke off from network TV, they broke off from casual viewership. Do you think the NFL wants to break off and cater to only "hard core" football fans who have to basically be in the "know" about this week's big game?
Further, individual local affiliates of those networks are resourced to hook up with local businesses for ad space. The NFL would suddenly need to hire ad salesmen in 50+ markets to accomplish what stations already handle. What you think is only 15 games is really about 50 channels when you consider the local affiliates.
And then the economies of it, why would the NFL want to pay every single different satellite or cable provider in America for 50 channels of which each one will only be used for 4 hours a week.
Its just not practical.