When you have the amount of talent those Cowboys team had, winning regular season games doesn't dictate success. You had a top 10 QB, arguably top 5, a top 5 receiver, a top 5 defensive player in Micah Parsons, good running game, great o line, one of the top young corners in the league. Definitely should have had more postseason success considering the roster they had.
And every one of those players have been injured for varying lengths of time. Go look at the list of injured players for any of the playoff games and you'll see at least one of them unable to suit up for the game.
And Dak has been terrible in high pressure games. He chokes more often than not.