As someone pointed out, in 13 games that he's started and NOT had McVay as his coach, his record is 0-13.
We can all guess as to why any player looks good for a couple of years and then goes south.
I don't know if it's fair or not, but the timing of Goff getting his big 2nd contract and his plummeting sure looks suspicious to me... and such a thing is not uncommon. He may have just gotten comfortable with his wealth, and decided that was the whole point of playing football anyway.
At best, Goff just never progressed mentally, and teams finally had enough film to understand what McVay was covering up. In other words, Goff's flaws were exposed, and they were many.
And, his long throwing motion just adds to the problems because the ball is exposed for a long time.
My opinion is he'll be out of the league in another year or two....retired.