There have been some attempts to convert office buildings with large rectangular floors into long, narrow apartments so every apartment has a window. It's possible, but difficult.[1]
Plumbing and sewerage turns out to be a huge headache. Large office buildings often have all the plumbing and sewerage in a small vertical core. The rest of the building is just flat slabs on columns. Adding a sewer line means punching through the floor and hanging pipe in the space above the apartments below. If you're in SF and want to see what that looks like, park in the 4th and Mission garage on the lower level, where you can see the plumbing from the restaurants above hanging from the ceiling. Also, sewer lines are gravity fed, with a 2% slope typical. Long pipe runs get lower along the run, so you probably have to put them along a wall. Then you have to hide and soundproof that stuff, although you might be able to get away with leaving it exposed if you market to hipsters or sell it as low-income housing. If the original building has enough ceiling height, it's easier.
Then there's HVAC, exhaust ductwork for kitchens and bathrooms without windows, partitioning the electrical distribution for the individual units, fire breaks between units, etc. Overall, it's maybe 30% cheaper than a new building, and all custom work requiring experienced people. If botched, it can be more expensive than a new building.
One company that does such conversions admits they're building tomorrow's slums.
And then there's the fundamental problem that if jobs are leaving the downtown core, why have more housing units there?
[1] https://www.pbs.org/newshour/economy/analysis-heres-what-it-...