You're missing the fact that these teams are starved for winning and that his mediocre is better than their best.
Also missing the fact of what it means when your team is either rebuilding and still has old slow guys (like 2021) or severely injured (2019).
The idea that Belichick lost his fastball in 6 months (2018 season when he won the Super Bowl by only allowing 3 points to one of the greatest offenses in the history of the NFL to the 2019 season when they went 12-5) is, frankly, preposterous.