There's only 16 above-average starting QBs and the great ones tend to last for a long, long time. So you're looking at 1-2 QBs who even become above-average, let alone great, out of every draft class.
You have to take in an unreasonably dense amount of information with a wall of large men in front of you and *instantly* know where the ball should go on and in what timing and you have to shift in the pocket to the right spot to do it.
NFL plays are freaking *complicated*. Most of them have some sort of check where they can be switched to a different play at the line depending on the look the defense gives. Many of them involve different sets of routes in different parts of the field and it's up to the QB to decide which set of routes to read their progression through. And the routes themselves often change post-snap.
Every time you get something right, defenses are taking note of it and will be trying to fool you by presenting you that same look but finding a sneaky way to take away the thing that worked.
I believe the reason so many QBs bust is that it's just impossible to tell who really has that ability to process information of that complexity at that speed on the field until they get here, because college football is just nowhere close to replicating the experience.
I've been watching a lot of Caleb Williams video and I totally get why the scouts rave about him, but a lot of the stuff he is rewarded for doing will be punished severely in the pros. He will have to reinvent himself and adapt If he can, you have Mahomes 2.0, and if you can't he's a bust and you wasted your pick and 2-4 years. And that's true for every other QB you could draft.
But if you can find a QB who can do it, it breaks the sport. Every offensive play is entirely in their hands, and they can make up for almost everyone else's shortcomings. If you take out their first year as starters and their last year before retirement, Manning, Brady and Rodgers played 50 NFL seasons and 49 of them ended with winning records. It didn't matter who the coaches were, or the offensive line, or the receivers, or the defenses.