Motocross seems a bit different than other sports to me. I played 3 years of college basketball so I got a lil exp with other sports. For me, motocross takes so much mental concentration that you are not focused on how you are doing physically until all of a sudden you realize you can't stand up for the 50 ft tabletop you are approaching in 4th gear. I usually have no idea that I'm getting tired. I just crap out suddenly because my mind is constantly scanning everything thats up ahead. This may be do to my lack of exp. but this one aspect has not improved even though my riding has improved.
In basketball there are always breaks, timeouts, fouls, whatever to get your breath back. I don't think it required nearly as much concentration because its just you and the other guys. Motocross is you , the bike, the track and the other guys i no particular order cause anyone of them will take you out.
I'm not saying that this makes motocross more difficult just more unique.