hi silver,
well, being that a drummer has got only two arms (i'm not going there...), he or she usually can't or doesn't play the hi hat while he's going across the toms or wile he's doing a snare roll. he also usually can't or doesn't hit the hi hat at the same time as the crash or ride.
think about it like that... he's only got two arms. although, when a really talented drummer is doing crossovers and stuff like that, you can watch in disbelief wondering how they can do it all with just two sticks. i don't think anybody wants to try and program those kind of drums in the piano roll though.
the allman brothers and a couple other bands have two drummers, that's something else, but they usually play as one anyway, just bigger.
hth.