The reasons I've seen cited, chiefly individual unit plumbing, don't jive with modern building construction.
Most office buildings are reinforced concrete floors, supported by internal load bearing columns, with glass facings.
... that's exactly the same as a hotel.
Hotels have per-unit bathrooms, typically stacked on top of each other (floor over floor).
Ergo, you can frame out/lose some space to cut several hotel-like vertical utility paths, plumb pipes from them to each floor's bathroom (or HVAC), and Bob's your uncle.
Actual reasons would be things like office floors are built to lower load bearing standards than residential... which I don't believe is true.
I found some papers [1,2] which claim conversions are not prohibitively expensive, though I assume not all office buildings are equally suitable. Open-plan office in particular seem impractical to convert. I wonder how many of the companies that built them that way to save money are now stuck with an empty building no one wants to buy?
