This is largely how open source game engines like OpenMW or OpenTTD work: the game engine is reverse engineered, and the art is something the end user provides by downloading/owning a legitimate retail copy.

And that’s really great, but this model is ultimately not realistic for most game developers.

It’s not like productivity software where the code of the product isn’t the majority of the value being delivered. Gitlab is happy to give away their source code because a bunch of enterprise integrations, support, cloud hosting, and features are paywalled.

Game developers really just can’t do this model. If the game is open source it’s going to be far too easy to pirate the game. The economics of single player games largely revolve around the strength of sales in the first month or two.

This model works for games on GOG because they tend to be priced so low that most users are okay with paying for convenience. Many of the games in that catalog are essentially back catalog that have been paid off for years and whose sales are quite insignificant to the publisher.

For a AAA game where it needs to sell millions of copies at a high price to break even on its huge production budget, game companies can’t risk a high piracy rate. Just look at GTA 6, a game with a production budget of multiple Avatar films.