This is an easy question. It's because they're cheating. :)
1) a mastermind coach
2) great personnel management
3) supremacy in the trenches
4) a wine and a nod from the league
5) an above average and intellectual QB that was make for their system
Why not set the trend instead of constantly adjusting to it? I know Adam Gase is a bright coach and the main reason I think he was hired was to match wits with Bill Bellichick who is known for his defensive brilliance.
BB is also the creator of the offensive scheme they play. Who better to design an offense than a defensive mastermind? He's spent a lot of picks and FA's trying to put together the dominant 2 TE offense.t provides so many options with the same personnel on the field.
It took 10yrs plus to get there, but they've had Gronk and the murderer, now Bennett.
Another thing, Bennett was quoted on media day, wondering why they continued to run sprints up hill and play in pads since xmas. The reason was evident when ATL D got gassed and the Pats seemed to not feel it at all.
All true, plus their overall coaching staff and the work they put in.
Check out this article: So you want to work for Bill Belichick?
http://www.masslive.com/patriots/index.ssf/2016/12/bill_belichick_patriots_coachi.html