On top of what everyone else said, just "spinning" up and down infrastructure isn't quite so easy. Servers aren't designed to be turned on and off, the boot-up process is usually the most damaging thing that happens to servers over the course of their lifetime - hence why most companies will leave them on overnight, holidays and extended periods out of the office where the applications or services they provide aren't being used.

And if it isn't your own equipment, then you are using a cloud provider. Everyone is going to want that service at the same time (it isn't three hours across the week, it's one specific hour three specific days a week). That means that either the cloud providers will need to invest in enough power to satisfy that 1/24th of the day that is peak timing over the weekend, or they just aim to satisfy the 23 hours of reduced, regular traffic and tell the game companies that the servers being unable to keep up with demand is their problem.