Good question you ask, I have wondered the same thing many times while running the combine. 

Perhaps one reason would be dumping the combine hopper, which was gravity.  If you look at the pictures of the IH 123SP combine that 1206SWMO posted last week, you see there is a trough on the side of the hopper that would be lowered to hit the truck or wagon, and a lever to open the flow of grain.  The operator would need to be close to that control to dump grain, which happened rather often with such small hoppers. 

Then again, that place for an operator is traditional in the United States, could have been habit.

Not to hijack the thread, but I have also wondered why, when we had gravity hoppers on combines years ago, someone did not think to put wheels under one and use it for a gravity wagon before the 1950's!