Well, Superman is supposed to be so fantastic that despite being able to crush mountains, he can pass for a regular man day to day in crowded offices and urban areas. As his powers increased from presumably the level of a regular baby to whichever points they really were with the bull and the truck, it's possible that he exhibited greater sensitivity than his alternate versions and that playing with regular humans so often actually helped with calibration.
As for why he would do it, there are more reasons to play football than physical challenge. I actually dislike the sport but my crush was into it, so I squeezed my dumb head right into one of those helmets.