You're right. Football is a team sport. That's why numbers don't mean nearly as much as success in big games, and only Montana can better Brady in that regard. It may be a team sport, but not everyone on the team has the same influence over the game's outcome: the QB has more sway than any other single player. As good as Tarkenton was, he still came up short in the biggest games. It's not a slander against him, but there are plenty of QB's who didn't come up short.
It's the nature of the game and of the position. Just like pitchers in baseball. No one wants to hear how good a pitcher's numbers would have been if he had better run-support. Why? Because if a pitcher loses 1-0, what does that really mean? It means the other pitcher threw a better game and the losing pitcher was simply out-dueled. Brady doesn't get beat in the big games very often. Since it's a team sport and the QB is the most important position on that team, who wins and who loses is THE most important statistic and THE most important talent is the ability to win the big games. Tarkenton didn't do it, Brady does.