The reason they cost so much even if no one is in them is because of what a "bed" means. It isn't the literal bed, it is a unit a treatable/treating capacity. Requiements vary somewhat by jurisdiction but it's going to mean 24/7/365 nursing and attending doctor staff. You can't just call them in when a patient shows up, they need to be scheduled and available. Then ICUs will also need a large cadre of oncall specialists, neurologists, cardiac, laboratory testing staff, and on an on to cover a huge range of possible patient needs. Stocked blood units, stocked medicine units. All those things have costs whether a patient in in the bed or not. Hospitals to a large extent spend an incredible amount of money on capacity. No wants wants to end up in a hospital to have them say, "oops, we didn't expect your spleen to rupture today, Dr. Bob won't be in till next tuesday so you are out of luck, sorry"