We can look for reason after reason. At the end of the day, it's regression to the mean. Fitz is an NFL veteran who was a subpar/borderline starter before he got to Buffalo, and that he remains. Each one of his seasons bears this out both statistically and on a more abstract level of general efficacy.
And hey, he shouldn't apologize for that. The fault lies in the Bills who decided the absolute pinnacle of his effectiveness was a good time to negotiate. These weren't the same exact guys who extended Dick Jauron after 6 games, but "he who doesn't learn history is doomed to repeat it."