I'm a bit out of my element here, but my guess is that's because success at the QB position is a function of so much more than physical talent.
Balancing risk-reward, processing defenses, making the right adjustments, commanding the respect of the team, and doing all that every 30-45 seconds or so, 70+ times a game. And doing all that in your late teens / early 20s.
To be a championship QB, the physical talent is a 100% requirement. But it is nowhere near sufficient.
Put another way, you must have the athletic ability. But even if you do, that only gets you in the conversation. There are lots more tests to come, and they don't have anything to do with release times or ball velocity or 40 yard dash times.
I think this is where the problem comes. Coaches can measure a lot of things. But they can't measure the intangibles that make the difference between, say, Blake Barnett and Andrew Luck at the same point in his career.
The only position harder to predict is placekicker. And we know a lot about both.