Why have the advancements in training, diet, recovery & injury helped him so much but nobody else? What he's done really is remarkable.
Why is Lebron the only player who has been able to play at this level at this age? Why was Kobe able to play as long as he did and play through the injuries he played through?
Both of them were known for spending millions on keeping their bodies in peak condition year round.
The difference between them and other players is that they were willing to spend that kind of money and put in that kind of work. Most players aren't willing to go to the extremes Kobe and Lebron have gone to. Brady is that level extreme.
It's also much harder to see any decline in an NFL QB than in an NBA player because athleticism is far less necessary to play QB in the NFL than to play any position in the NBA.