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.

Good construction is not cheap and takes many quarters. Land outside the urban area is far too cheap and probably subsidized (directly or with free oversized infrastructure) because local government always wants jobs, even small numbers of shitty jobs.

I thought about this a lot with parking spaces, nobody like big, open, tree-less parking lots. Why not just build them up adjacent to the grocery store.

The answer is $. It costs too much and land is cheaper than building up (In most places).

I would guess that having everything on one floor optimizes the unloading, restocking and logistics. Also construction is cheaper.

The vast majority of the inventory is already on the sales floor.

Also, the backstock is minimal. Stores are designed for turnaround.