Do you think that the setting, themes, story, and RPG rules are just 'paint'? Is the Infinity Engine so much more important than the Forgotten Realms?
> I also don't see why games should be inherently different from other media in this respect.
Because games aren't media: games are media generators. The same game can be a tragedy to one player and a comedy to another. So you can have two different kinds of follow-ups: ones that simply generate more media in the same way ( = expansion), or ones that improve on how the media is generate ( = sequel). The distinction doesn't make sense for fixed media like novels or films.
Note that I'm not talking about video games in particular, it applies to any kind of game. If I publish a book that adds a new secret level to the crypt of Acererak in the same style as the original, it's an expansion. If I publish a story about a different crypt that feels connected to the original but with its own unique design choices, that's when I can call it Tomb of Horrors 2.