While your premise is correct, the assumption is wrong.
If Mark Rypien and Joe Flacco and Brad Johnson, Nick Foles and Eli Manning can win Super Bowls, so could Tony and Dak.
The makeup of teams, coaching, scheduling, playoff schedule, weather, calls and just breaks all converge. How do you explain the 49ers dropping TWO punts that led to Giants Touchdowns in a playoff game that helped them get to the Super Bowl, never mind the ridiculous Tyree catch.
Assigning blame to QB's is easy and we all do it (yes, me too) and it goes with the position but at the end of the day, it's not fair.