Because Brady understands that if he demands a mega contract like Stafford got in Detroit and Rodgers is about to get, the Patriots won't have enough cap space to surround him wit the talent he needs to keep winning.
For 2018 Tom Brady's cap number is $22 million compared to Rodger's $20.9 million. The Pats' cap number in 2018 for top 50 players is$163,052,002 compared to the Packer's $160,443,759. Where each organization spends is fairly different when you compare the caps spots. I used Over the Cap for the data and the Packers are top heavy for the top 10 contracts and spend far less on the remaining 40 spots. Just based on numbers it looks like the Patriots are more balanced and less reliant on their top players.
Patriots Packers Cap Spots
$96,317,438 $114,294,500 (1 to 10)
$34,341,666 $26,748,865 (11 to 21)
$17,767,701 $7,696,151 (21 to 30)
$8,559,635 $6,229,243 (31 to 40)
$6,065,562 $5,475,000 (41 to 50)