What does it matter? Trent Dilfer has a Super Bowl ring. They didn't win because of his QBing. Even if you wanted to use intangibles as an excuse, he wasn't the leader of that team, nor did his ice veins make a difference. Super Bowls shouldn't be used to measure an individuals career. Only stats and individual achievements can measure that. And Manning's trump all.
You asked why do QBs get the most credit in team sports. I simply asked if you ever tried to play the position? Implicit in the question was the suggestion that the role in the overwhelming majority of cases makes the biggest difference with the success or failure of most football teams.
Are there exceptions? Of course...and that's the reason why you don't usually see average or below average QBs achieve consistent success.
Given the right circumstances and team...almost any QB who's made it to the pro level can win just one.
Brady was a fluke winning 1 in his first year starting. Then he won 2 more in the next 3 years and been to/could have more.
One thing you'll never see of Brady is the pitter-pattering nervous wreck in the pocket that Manning was throughout his career.
I'd love to see what Manning would have amounted to if he ever, just once had to manage without teams built around him with nearly every phase of the game capable of wining games for the Colts.