Not playing other teams in your own conference is pretty head-scratching. Texas A&M has been in the SEC for 10 years and has played Georgia once in that time-span. I know they play more for basketball, baseball, women's sports, but for football that is almost like not even being in the same conference.
Also since the Big 12 will likely not be getting multiple bids regularly to 12 team playoff, why should members stack the odds against themselves to win the championship and get the sole auto bid to playoffs? Better to have a 1 in 9 or 1 in 10 chance than a 1 in 12 chance or a 1 in 14 chance.