I think it just speaks to the enormous difficulty of the job. It's funny because if you lose a starter in the NBA, the backup will almost always be a reasonably capable player. Not to say there's no dropoff from LeBron or Steph to the next man down, but those guys will at least be able to play something resembling winning basketball. The same idea applies in other sports like soccer. Messi's backup for Argentina isn't Messi, but will still be a good player.
On the other hand, there are about 10-15 guys in the world who can play QB in the NFL and everyone else is effectively awful. I don't understand it, but we can probably conclude that the job is just insanely difficult to execute. You'd think some of these great college QBs would be able to play serviceable football in the NFL, but they just can't. They can't read the field and make the throws fast or accurately enough while coping with the pass rush and speed of the defenses.
I will play devil's advocate here.
There are more of these guys than people think they are. They're just not the guys that scouts and coaches care about.
I think most here have seen my story from the 150yds thread. If you're not 6'2+ with a huge arm, you're just not of interest. Obviously occasionally a true standout can break through. But the guys that aren't big with a cannon just get weeded out further and further the more you progress up the ladder towards the league.
The problem is those big guys with big arms get by on their athleticism for so long, they never learn how to be a real QB until it's too late. Obviously the good ones do, but the guys that are going to end up battling for a 3rd string NFL QB job just don't, and continue to get strung along based on the hope they'll one day put it together while the normal athletes that could develop into a completely capable 3rd string NFL QB get left behind.
I'll share one specific example I know very intimately, working closely with the UF program back when they were good. I worked with the team and was also friends with the 3rd string QB at the time, and helped him train. With all love for my buddy, he was a truly AWFUL QB. But he was 6'3 and could throw the ball on a freaking rope. But just awful. After he graduated and stayed in school for grad school he played intramurals and I'm not exaggerating when I say he was not a top 50 QB in the UF intramural flag football league.
I remember him eventually throwing 5 INTs of the first half of an intramural game and getting benched. But yes, back when UF was competing for BCS National Championships their 3rd string QB was a guy that wasn't good enough to hold down the starting job on a middle of the pack intramural flag football team. And there is a scenario where he could have played, on national TV, in front of millions of eyes, with millions of dollars on the line.
The scouts always always always shoot for upside. But there is an endless supply of crappy Drew Brees's out there that could be a mediocre 3rd string QB without future starter upside.