I think part of it is that he mostly played at the tail end of an era before the rules changed to help offenses be dominant.
In 1978, the year the most major rules changes hit, he was second in TD's and third in passing yards one year removed from retirement. He had even more of each in his last season, and led the league in passer rating both years (more than 50 attempts).
Yeah he had a great team around him, but it wasn't even his stats but his leadership that made him great.
He believed that no matter what happened or circumstance he could always win the game, and his team believed it because he did.
I loved that guy.