The weird thing is that you’d expect that skill could be learned over time. Maybe his problem was being a “starting quarterback”. If he was looked at more as a rookie who needs to be “brought along” maybe he’d end up being great somewhere.
Learning to read and react to defenses while in the pocket can be learned.
What can't be learned is the ability to process the info in 2-3 seconds.
Some can, many can't. Its why a college QB can win the Heisman and get into the NFL and suck Balls. In College most of the high end QBs face at best two or three defenses in a season that have 3-4 players (or less) that would play first string in an NFL defense. Those players are very strong and most are Olympic Sprinter fast.
An NFL QB plays 16 games in a regular season and every game has a defense they are playing against in which 90% (or more) of them are strong and Olympic Sprinter fast! (Even the 350lb. lineman are freaky fast for 10 yards!)
2-3 seconds read and react time is crucial. And the list of the members of that particular club is very short.