Moreover, no one is converting these unused offices in to residential properties. I’m guessing because the average price per square foot is significantly lower for SF property targeting residential use.

That's part of it, but there's also significant up-front costs to doing that type of conversion. If enough spaces become apartments, the remaining commercial space will profit without needing nearly as much investment, so the property owners are in a bit of a game of chicken to see who will break first and convert or sell low and start tanking the market.

I read a thing where they can do this with commercial buildings from before like 1950, but a lot of the big skyscrapers are harder. The older buildings have a smaller footprint, so it's easier to convert them to apartments with windows and a decent layout. When you convert the big buildings, you either end up with a bunch of really long, thin apartments with a window at one end and no natural light in the other rooms, which makes them unappealing; or you get a bunch of apartments clustered around the edges with all sorts of unused space in the center. You can convert the center space into functional space - storage units for the apartments, a gym, meeting rooms, etc - except then you're committing to higher costs for maintenance, cleaning, insurance, etc.

Personally, I'd like to see the second option become a thing, but then I don't own any large commercial real estate, so ...