Why only one bed in a fifth wheel and why sooo tall?

They are taller because of their design. Makes the insides feel much bigger given the same space.

Class A's have the storage below which isn't as tall as a fifth but extends down the coach so having that cuts down on the ceiling height with the max vehicle height being the same for both.

If you want more than one bed there are plenty of floor plans from different manufacturers that have all sorts of bedroom combinations. I'd say if you have only seen one bed then you haven't looked hard or long.