Yeah, the skills required are different. If American football players had to run the entire game, they would be much smaller than their current sizes. Currently, the weights and calorie regimens help them bulk up. With that bulk, they can sustain great power and speed for short bursts. But the average football play is only a few seconds long. In the typical NFL game, players are in motion for 11 minutes and most run 1-2 miles a game. Conversely, rugby is played constantly with the players running around 5-8 miles in a match. There is no way US football players' bodies could support that mass and have enough fuel to play a continuous game for 60-90 minutes without much in the way of breaks. It is more like sumo wrestlers built for short violent collisions that require mass and some subtle coordination. Of course, those same athlese in the NFL could reengineer their bodies for rugby and would likely perform very well.