First off, the Big 12 historically has terrible defenses. Second, that reputation isn't going to change until a Big 12 team actually does something on a national level - like beat Alabama or Georgia in the playoff. Every single time a Big 12 team (Oklahoma) has gone up against an SEC team in the playoff the defense has been as bad as advertised, and the offense isn't near good enough to break an SEC defense enough to keep them in the game.
Of the grand total of TWO times it’s happened, one was a 2OT loss and the other a 9 point loss (albeit not that close). Both at the hands of OU who had an exceptionally bad defense.