That's the unintuitive part. An empty building still needs some basic maintenance, and the financial difference is almost immaterial. Else the building just falls into disrepair and crumbles. The cost of conditioning a building for human use after dereliction is way larger than the cost of keeping it maintained, but the cost of maintenance between empty and full is almost the same. You can keep the AC off, you're saving that electricity, but you still have to pay for the technician to go there and make sure it is still working, same with elevators, water lines and electric networks. Those things still deteriorate even if not being used. Some things can be mothballed, some can't.