The short answer is simply that athletes hone themselves for the specific sport they play.
It's not just a case of getting punched and mauled, MMA fighters train to fight for 25 minutes. If they can fight for 4-5 hours my guess is they would be too physically weak, having sacrificed strength and explosiveness for cardio they don't need. This would be a mistake in MMA.
Of course we do see a disparity in cardio amongst MMA fighters. There are guys whose cardio is perhaps "too good" and it comes at a sacrifice. The Diaz brothers, for example, have great cardio but lack the physical strength of many other fighters. Guys like Lombard are at the other end of the spectrum.