Also, the plumbing isn't suitable. Office buildings have all the lines routed to a few huge bathrooms per floor, plus maybe a standard break room that's in the same place on every floor.

An apartment needs its own bathroom, its own kitchen, etc. So, instead of the pipes going to two areas on each floor, they now need to be re-routed to go to 50 small areas on each floor. So, the entire plumbing for the building needs to be redone.

Also, modern office buildings are very big, and there are a lot of cubicles / offices / open areas that are nowhere near a window. When the place doesn't have many walls, maybe that's not so bad, because you can still vaguely see a window off in the distance. But, when it's an apartment building, you can't now have an apartment that doesn't have windows. So, the apartments have to end up being these narrow things with windows only on one side at the end.

Big windows, but only on one narrow bit of an apartment is bad for ventilation, and it also means the place gets really hot when the sun hits those windows. When the thing was an office building the entire floor was one "climate control area", with a limited number of walls and lots of wide corridors. So, if it got hot on one side it would even out across the floor. That doesn't work when that floor is subdivided into 50 individual apartments. So, now you need brand new solutions to the problems of ventilation and climate control.

It's less work than building a whole new building from scratch, but there are a lot of trade-offs.