because the way they train and the nature of their sport is anaerobic. They are not running 10miles or doing extended periods of aerobic activity during game time. Look at how long a football play lasts, 10 maybe 15 sec. it that. Sprinters also, think about how long it takes those guys to run 100 meters. By the nature of their sport they are basically doing HIIT all the time. This is how most of them are so lean and muscluar. Yes their VO2 max needs to be high. But long, low intensity cardio is not the only way to increase VO2 max. It can be raised with High intensity cardio (sprints etc.) as long as you are raiseing your heart rate up to about 85% of you maximum heart rate and the sustaining that for about 60sec. or more. This is possible for football players, look at their practices or just being on the fiels play after play. They are really elevating their heatr rates. Not to metion just the running they are going but all the physical contact and force production needed during one play is really going to get their heart rate up.