Apartments are quite difficult in the footprint, I think. Depending on the size of the unit, it becomes impossible to give everyone a window, and not have some overly narrow floorplan. And that's not even taking into consideration the need to build out the plumbing to some x10 or x50 scale of what it currently is. Every unit needs a wet wall for kitchen and bath. Evacuation routes suffer.

Retrofitting should certainly be possible, but neither cheap nor quick, and you either run into the problem of the resulting monthly being too high (and having trouble getting occupancy up above 95% where you want it), or it goes too low and you can't make back what you spent,