Well, the odds are that most QBs don't make it.
In any given class, if 8-10 QBs get drafted, 1 may be a great all pro, 2 may be solid starters, 2 may hang around as backups...the rest probably will be out of the league in a couple seasons.
Again, he's probably a 3rd rounder, but you do not want to be the GM who could have drafted him, especially if your team needed a QB.
A couple seasons down the road if he's a bust the owner won't get mad at you for drafting him. Real simple "hey boss, kid had too much potential to pass up, we needed a QB and selected the biggest, fastest, most athletic, biggest armed one..."
Infact there are probably multiple owners pressuring their front office to take him. Owner's love bright shiny projects with potential.
However, it's harder to explain why you didn't take the 6'4 tank who moves like a Porsche, who is currently looking pretty good on some other team.
That's a different convo with the boss.
"Hey dipshit...remember that real big, fast, athletic, strong armed QB from a few years that you didn't take...well he's tearing it up right now...what was your reasoning for not taking him again..."
"well boss we really liked the rangy CB with long arms..."
"yeah...cool...you're fired..."