Or at least A reason ...
Brady is worth 500,000,000 ... However his salary is below 1,000,000.
His $$$ comes from being unbelievably successful and the marketing deals that come with at success ... He realizes that his salary cannot hold the team hostage because it would threaten the power of the team and thus his success.
Add to this the fact that his wife is worth 1,000,000,000 and it becomes fairly simple to have a low salary.