Most games are online games now.
Multi-player games depend on servers, economies, and communities that exist only as long as someone's logging in and paying the bills. When those servers go dark, the logic, data, and social fabric disappear with them. Owning the client isn't owning the game. The real thing was the networked experience -- like trying to preserve a concert by saving the recording but not the crowd.
Legal and technical barriers seal the deal. Server code is proprietary and rarely released. Reverse-engineering it breaks copyright law in most countries, so preservation sits in a gray zone. Even when fans rebuild servers, the result is closer to a reenactment than a recovery. And they seldom scale to the heights of the actual game.
Future historians will have YouTube clips and half-working emulators -- fragments of worlds that were never meant to last.