QB's coming into the league are tossed into the lineup with zero developmental time. It wasn't that long ago that a team would draft their QBOF a few years ahead of their franchise vet's retirement. For example, Aaron Rogers sat behind Farve for a long time before Green Bay moved on from Brett.
If a QB coming out of college is lucky, he ends up on a team with a coaching staff that matches what he accomplished in college. KM is a good example---his pro coach actually went in reverse and matched the playbook to what his new QB brought with him from college. If not, KM would be a disaster---even now, on a lot of plays he is staring down his primary receiver and if that is not there, he tucks and runs. Imagine if that play was from a brand-new playbook? KM would still make some plays based solely on his athletic ability---but he would also have plenty of INTs and misfires as well. It also helps if the QB goes to a team with some talent---who on the Giants matches up with Hopkins and Fitz in the WR room?
QBs have almost zero time to study NFL defenses before they are asked to go out and perform. When QB's sat, they had the opportunity to see what a given D was going to do. Now, it is learning on the job.
Teams want immediate results. The Cards may have provided a new blueprint for teams and QB/!st round picks. They took Rosen and after one year went QB shopping for both a new coach and new QB. Rosen went from 'most NFL-ready QB' to sitting in the stands behind Tom Brady. Look for teams to move on from 1st round QBs when they don't come in gangbusters like LJackson and KM. If the Giants have a chance at Lawrence this draft---pretty sure they will pull the plug on Jones.
