I imagine there's a lot of confirmation bias. Coaches and players will say "yeah I could tell". But most rookies aren't great and do improve and I imagine unless they're a complete disaster it's hard to tell.
Tyreek Hill said Mahomes was trash as a rookie and imagine people have thought that about various rookies who've been good just don't then come out and say "yeah I thought he'd suck"
They've done this a bit in rugby and they tend to be worst refs. Not saying PGMOL have done a good job with a lot of their decisions but the job is so hard to do without mistakes whilst the criticism is so high that even if the quality of onfield reffing improved I don't think people would have confidence.
It's why there should be a focus on improving VAR to make it a streamlined and consistent process which PGMOL and everyone else has made a meal of