I can see why that argument might work for a laissez-faire libertarian, but we have innumerable laws on what is contractable. That this should be singled out specifically as something which shouldn't have any such protections seems in my experience to be attributable to the fact that people don't consider games important.

Besides which, I challenge your claim that the buyer knew the terms perfectly well. The terms are often dozens of pages long and written in dense legalese, with clearly little belief or intention that the buyer will read them, and arguably specifically to dissuade the buyer from reading them.