Because Brady was 44 and played at an MVP level and won a Super Bowl at the age of 44 (and then was really good and made the playoffs at the age of 45), people think that Rodgers will be good at 40, and Cousins will be good in his mid-30s, or Russell Wilson thinks he can play another 5 years. In reality...throughout the history of the NFL, QBs really start to fall off between 35 and 38. Look at Steve Young, Joe Montana, Manning, Manning, Flacco, Marino, Elway, Brees, etc. etc. Can QBs have good games or even good strings of games at age 37 or 38?
Sure.
Can they even have good seasons? Yeah actually. Brees was good at 39 but fairly limited in the throws he could make. But then he couldn't throw downfield at 40. Remember they would bring Jameis or Taysom in to throw downfield because he couldn't do it.
My point is that Brady has fooled NFL teams into thinking Cousins can be good at 35 after an injury, or Rodgers can be a franchise guy at 40, or Russell Wilson is a long-term solution for the Steelers, etc. etc. What he did will never happen again (being ELITE at 44). And even being consistently good at 37 will be rare.
Allen is an athletic freak, so maybe he can be good longer. But Mahomes' ankle and leg injuries are starting to pile up and he's only 29. To think he'll play this way for another decade is silly. Same with Lamar who relies on movement so much. These windows are shorter than most assume.