Any service could leverage their pre-existing DRM scheme to let you sell your copy to someone else

But this would not be interoperable with other services, and to make it interoperable would require mutual trust and coordination between them that does not exist. Not to say that NFTs specifically are a solution here (video games are fungible after all, and porting physical ownership properties to digital goods is a problem these companies don’t want to solve to begin with), but there are actual reasons to use credibly neutral infrastructure and standards.