The cynic in me would say that it's because lower stands cost less to manufacture and ship.

Don't forget the outside for storage. Plywood boards hung on the outside are handy for collets, clamping kits, end mills, etc. and can be made so that they can be easily removed and taken to the work, then returned when finished. I spritz mine with a diesel fuel and ATF mixture to stave off rust and occasionally brush the swarf off.