Probably because you then need pillars throughout the entire building to support the second floor which you are loading down with a ton of weight. The average forklift weighs 3x or more the weight of the average car, and then adding racking and stock on top of that. Yeah if you completely redesign your storage system to not require forklifts you save weight there, but you end up adding the weight back with all the heavy duty track systems and extra heavy duty racks that are required to eliminate the forklifts. Plus there is liability of having that weight up top, a rack failure on a second floor could take down half the building.

It is possible, but you end up spending 10x as much on the building.