You’re right, and you’re pretty much describing the sunk cost fallacy as well.
They probably don’t save much money that way though. You stop have to keep the place warm enough and clean enough to prevent mold and frozen pipes. Many are in bigger buildings with cleaning, maintenance and security contracts.
I suspect as dumb as it is, the moronic managers still think micro-managing makes up for the 5-10% cost of employees there (might even be less in some offices). And as said before, they usually at best break even, while alienating the smarter employees who go find wfh jobs.