Property owners are terrified of this becoming a go-to strategy, particularly funds that might have investors breaking down their door to get it to happen ("Isn't some income better than no income?"). Residential units go for less, and even an inexpensive conversion is a capital investment of some order, so you're putting money into the building to devalue it. Alternatively, you could not do that, and your balance sheet survives to see another day. Who cares about pathologically inefficient use of existing infrastructure?

Right.

My first thought: find an all-but-vacant office building, build myself a sweet pad on my favorite floor, then go about slowly converting the rest into apartments/condos while reserving some floors for business - esp the ground floor.

Good luck convincing the city to rezone it

The 10 foot ceilings are too low for the modern renter's sensibilities, apparently.

That's... strange? I thought 8ft was standard for (non-"luxury") residential ceilings.

For many reasons this is very difficult and expensive.

Is it more difficult and expensive than finding new land on which to build new residential buildings in Manhattan?

No, because that is impossible since all the land is spoken for. But its so difficult and expensive that it is almost always uneconomical. i.e. you will never make the money back or the yield is so low might as well park it in bonds.