And why not? He looks awesome doing it. There are a lot of great running scenes in films, like the ending of "Children of Heaven" where a poor Iranian child competes in a foot race to win a pair of shoes for his sister (spoiler alert, only watch this clip if you've already seen the movie. It's on Netflix Instant right now and it's beautiful):
Another that comes to mind is "The Loneliness of the Long Distance Runner", an old British film about a troubled teen who finds solace in running while away at a boarding school. This shows it as a sad but contemplative pursuit:
But if running is so great to watch in movies, why isn't it much of a spectator sport? The reason is that the races that are the easiest to film are the most boring to watch. Marathons get some coverage on TV because that is where the prize money is at, but the race is more than two hours long which doesn't make it very exciting until the end. NBC Universal will air Diamond League track races, but watching men run in a quarter mile circle over and over is only fun if you understand what the times of each lap mean (I get pumped watching Galen Rupp knock out a 52 second final lap, but that's only because I have done repeat quarters myself and realize how fast that is.)
Of all the running events, the Cross Country is the most exciting. Pushing up hills, slogging through mud, or jumping over hay bales is way more fun to watch than someone running even pace on the road or track. And with that many people crammed into a tight space you see jostling and pushing for position around tight turns. Also, the length of the races are perfect for viewing: 20-30 minutes is long enough to establish a narrative but short enough to keep it interesting.
So why aren't we putting these races on TV? The unfortunate answer is the terrain makes it too expensive to film. If David Fincher shot a collegiate XC race with a Panasonic and full lighting, you wouldn't be able to take your eyes away from it. Running is kinetic and violent, vibrant and suspenseful. Anyone who has ever raced can tell you, but you will only believe them if you can see it for yourself.
However I think this could change soon. I watched a few XC races this fall and when the picture quality was clear it was really exciting. They had camera-men on four-wheelers following the action, and when there were close, head-on shots you could see the tree's whipping past them and you could actually get a sense of how fast the athletes were going. New digital camera's like Go-Pro's make it easier than ever to film action, and improvements in broadband internet technology should allow us to broadcast higher-resolution images from those cameras to the rest of the world. And if smart, charismatic commentators can explain the rivalries between the runners and the racing strategies as they play out then I believe more people would tune in to watch.