I'm not sure that's true. Admittedly, I'm not a scout but in the Michigan games I've seen, he's got a really good arm. And the scouting reports I've read on him agree.
My personal feeling, backed by absolutely nothing, is that QBs bust for a few reasons -
1) addiction issues - can't fix that
2) mental health issues - can't fix that
3) injuries - can't fix that
4) being in the wrong system/bad coaching.
I think #4 is the main problem. Freaks like Peyton Manning and Joe Burrow are going to be good anywhere but normal QBs, even upper level ones, thrive with good coaching and fail with bad coaching (Either being rushed or playing in an offense that doesn't fit them, sometimes both, etc). The one thing I think the Vikings have with KOC is a coach who knows how to coach up a certain type of QB (pocket passer, throws deep, keeps his eyes downfield). I think guys like Dwayne Haskins or Daniel Jones could (or Zach Wilson or Sam Darnold) might have been good NFL QBs in the right system/right amount of seasoning. So if the team thinks, "McCarthy (Nix/Penix) is a perfect fit for us", I'm ok with them drafting him. If it doesn't work, they all get fired anyway.