As far as I know, the biggest problems are converting plumbing, heating/cooling, and electrical from a communal (often open-office) setup to individual units.

As far as I know, the biggest problems are converting plumbing, heating/cooling, and electrical from a communal (often open-office) setup to individual units. Office buildings are often concrete with drop ceilings to make it easy to access sprinklers, wiring, and pipes, but you would still need to run all of those things across every floor of the building. A single, modern condo likely has 2+ full baths, so if you're trying to get 10 units per floor, you're going to do that work 20+ times. Kitchens are also difficult, yes. You need to be able to vent heat and smoke, and you need different electrical wiring in most cases. Then think about ceilings. Drop ceilings are not appealing to most people, so you need a solution for that. Even if you keep the drop ceiling and just use a nicer tile, you're still spending $5-$30/sqft. Walls aren't that expensive, but you need a lot of them to convert back from the trendy open-office plans that most buildings have. There's also a lot of legal work to convert to condos (assuming you don't want to do rentals, which most of these office owners absolutely don't want to do). There's just a much bigger difference between office and residential space than people realize, I think.