Because that's similar.
I have a friend who was an NFL Europe QB and he throws an amazing ball, effortless, like he's tossing a napkin in the trash.
The gap between good college player and good NFL player is enormous. Hell, the gap between GREAT college player and good NFL player is huge as well. Tossing a ball with the kid compares to making it in the NFL like walking outside and looking at the moon compares to being the first man to walk on the damn thing.
Or watching a hitter crush batting practice fastballs and extrapolating that he will be able to hit major league curve balls.
Whatever you do in the NFL at QB you have to do under ATTACK as in 300 pound plus guys all rounded up for their freakish size and speed line up every play to KILL YOU while you are asked to do your job.
You are asked to get past this, to stand there then hit a moving target that is being chased by some of the fastest most athletic humans on the planet who spend one week a year studying you sometimes two weeks a year in your own division, looking for anything you do that gives away what you are about to do, which is surprisingly a lot.
IMO being an NFL QB is the hardest thing there is in sports, there isn't really anything close to it most of it comes from how smart you are but only if you posses what amounts to about 1 percent-er type skills at throwing a football.
Kurt Warner wasn't great because he had a great arm, he was great because he put the ball where he wanted to put the ball and that is all done while people are trying to literally kill you every play.
He still could throw the ball better than any random sample of 100 guys though but yet in the NFL he wasn't anything special at all arm wise.