It's also a question of what the trunk water/sewer line can handle. There's places in some cities where larger apartment buildings have to hold sewage in a tank and pump it out at night because the city pipes can't handle a normal load during normal hours.
The bigger problem is finding space for pipes. Many commercial buildings have specialized vertical tunnels for plumbing. The bathrooms in these buildings are usually adjacent to these tunnels, and there are no provisions for expanding plumbing to other areas of the floor in order to separate the floorspace into separate apartments with their own bathrooms.
The vertical tunnels are also designed to handle some number of main pipes based on N bathrooms per floor that you'd typically find in an office, with no room for expansion to handle N*10 bathroom capacity.
Pipes dimensioned for an office floor is surely enough for flats.
Although showers could mess up my 'theory'. But the sewer from those is easier to get flowing than toilettes I guess.
AIUI, plumbing is technically easy, but is still insanely expensive for conversions.
That said, it makes sense that HVAC would be a bigger problem.
Plumbing is easy, pressurized pipes and pumps can deal with that just fine. The real challenge is air conditioning: showers create lots of humid, warm air - introduce that into your building's HVAC and you'll get issues with rust and mold - and cooking isn't much better, people generally don't want to smell the curry that someone at the other end of the building is cooking. Or they don't want to hear their neighbors having sex through the air vents.