First of all, because he can. Brady is worth an estimated $100 million dollars — wife Gisele Bundchen is worth even more — and most likely, on the heels of his success and good looks, Brady will be able to pitch products for most of his adult life.
The second reason he accepted at least $25 million less than what he could've earned is that Brady is aging. The 35-year old always has said that he wanted to do two things: Play until he was 40 and finish his career with the Patriots.
With this deal, he accomplishes both.
(Read More: The NFL's 10 Best Cheerleading Squads 2013)
However, the key here is how good the team is in that time. A lower salary for Brady means more money to spend on other players, such as his favorite target, Wes Welker, who is a free agent.
"Some of this is selfish, too," said Dr. Patrick Rishe, a sports economist at Webster University in St. Louis. "Brady loves tossing the rock to Welker. He realizes that his own success is enhanced by having familiar and dynamic players surrounding him."
That is even more important over the next few seasons because Brady wants another title, and there won't be any meaningful expansion in the salary cap. Without Brady taking a hometown discount, the Patriots would have been faced with difficult decisions.
One of them may have been whether to jettison Brady himself.