Why not? You might have to redo lots of phys work but essentially all of the RTL will be the same and that's the vast majority of the work.

Intel doesn't have demand because they only make Intel chips, and they haven't been doing too well lately.

They feed into each other especially for anything that isn't a vanilla gate. Got a deeply ported SRAM with bypasses? That might fail synthesis if it is too choked by wire rules for the size of the cells so now it's banking time.

Right, you might get a different PPA...

I think realistically you wouldn't port the exact same design between manufacturers. That would be a waste of money unless one manufacturer is really rinsing you.

More likely you'd switch manufacturers when you planned to switch process nodes anyway, in which case the increase in workload probably wouldn't be too bad.

I honestly don't believe that e.g. Apple couldn't relatively easily base their designs on a different underlying technology.

They do it all the time when they change nodes.

Drop another billion is sort of the name of the game here.

This. And the extra time and Human Resources required for redoing the design along with testing.

It is not that it cant be done. It is not reasonable or cost effective to do it without some clear incentive.