Why would networks overpay for TV rights to the NFL?

Just reading this in an article quoting from a book A Cognitive Psychology of Mass Communication by RL Harris, 2001. I found this very interesting:

"Four major networks paid over $21 billion to broadcast the NFL games from 1998-2005, even though the likelihood of recovering their investment with advertisements was minimal. They did this for a number of reasons: because they wanted to maintain a large viewing audience and their "major network" status and because they were afraid that without football programming their affiliates and marketers would take their business to other networks."

The book said that's what happened to CBS when they didn't carry NFL for a year or so, some of their affiliates went elsewhere. Man, you don't realize how big such a deal like sports on TV can be.