[bell-cot]

By the time you've complied with all current codes, stairs themselves do take up an unreasonable amount of floor space.

But there's a usually-worse cost to the second set of stairs, that I'll call "geometry". A second set of stairs is not a giant support column, which you can wall off and ignore. The halls, doors, and such that it requires also suck up space, and impose many more restrictions on the layout of the actual living spaces for human beings - which are the sole reason for the building to exist.