Think of the layout of a modern office tower. Nearly open floor plans, one or two multi-user bathrooms per floor, one or two kitchenettes per floor. Designs optimized for ingress and egress via a single path. Sound abatement is minimal. Support columns are in the open.

So at minimum, you would need to plumb and power each "apartment" which would require essentially a complete replacement of the plumbing in place, as well as the electricity. And since the units are distinct, you would need to meter and isolate each unit. You would need to ensure that each "apartment" had a reasonable exit for fire safety reasons. You are restricted by architectural/engineering decisions that were based on an open layout (aforementioned supports).

Think of it this way, have you ever seen the progress as a tower is built? Isn't it remarkable how the "shell" or exterior seems to get finished in a blink, but the rest of the "innards" take 4 or 5 times as long to finish? In this case, most of the "innards" would need to be re-worked. Huge capital investment with unclear returns (nobody wants to live among the blight that a lot of west coast city centers are seeing).