rft2800;
maybe it's because of the size of American dryers? Quite large. I think old Whirlpools did have a solid drum, and GE's.
When they began making them larger, it's simpler, and more easy to move air and heat through one large vent in the fixed back of the drum.
So they eliminated the center bearing, and set the drum on two rear roller support wheels, added a rear gasket, and two more at the front, and or, use teflon coated slider bearings on the front bulk head.