Office buildings tend to have all their plumbing centrally located. Converting to residential means spreading it all around. It's not like just moving partitions around.
Sprinkler systems tend to be evenly distributed around floors so the conduits etc. to run additional piping are already in place. Not every building has such systems but most constructed after 1980 do.
The sewer pipes and vents have to be run everywhere, and they have specific requirements. Sewer pipes, for example, half to have a slope. The vent pipes have to go up. There may be floor loading problems. There's going to need to be a lot more electrical wiring.