There are no air ducts to move air between the units. They are old brick buildings and retrofitting central air you might as well tear down and rebuild a lot of the time.
Which of course means they should use heat pumps, since the copper lines don't need nearly as much room (I suppose you'd still have to put a drywall wall in front of one brick wall potentially). Even just putting a bunch of mini-splits on the roof (instead of individual window AC units) would be an improvement, seems like.
The only difference between a heat pump and an AC is a reversal valve to change which direction you’re pumping the heat in. (Of course obviously an AC fundamentally is a heat pump, but setting that aside.) That makes it very hard to puzzle out what you mean by this comment.
Yes, mini-splits are a good retrofit option, and they are efficient as well. They are more work to install than a PTAC, though, so cheap landlords pick PTACs to reduce labor cost.
PTACs are self contained and so they don't even require an HVAC technician to install. Simply knock a hole in a wall and plug it in. They are basically a permanent version of a window AC unit.