Short version:
Commercial buildings are too deep and so you have to do fancy things to give internal apartments natural light and meet building regulations for windows.
Youtube link:
You can also search for the podcast.
A lot of times office buildings will have one large bathroom per floor.
The right question is "Is it easier than tearing down the building and building a new one that's designed for residential use in the first place?".
What they're saying is the buildout from commercial to apartments involves more work than the buildout from one commercial client to another.
Also if it is a tension slab you absolutely cannot cut into it.
Malls are also on the table for conversions these days. That is a much simpler transformation.