That's how most opensource game revivals work, one is not legally allowed to distribute original in-game assets. If you come up with free textures and sounds, you'll get the 100% free alternative

Thanks for your explanation.

The assets are still under copyright. On paper you could "re-implement" assets as well, ie. remake all the models, textures, music, voices and rewrite all the quests.

This is what the Skywind project is doing.

No, you have to create a new game based on the OpenMW engine. Kinda like FreeDoom for Doom.

Thanks for your explanation.

Because software copyright was and is a bad idea, and courts (especially the 9th Circuit) have generally carved out exceptions for compatibility-related copying in the software domain that absolutely would not apply to other copyrightable work that ships with the software. You absolutely can legally reimplement a computer program such as a game engine, but doing so to the story, assets, level design, or what have you would just be ordinary copyright infringement. As a result most game reimplementations either do not ship with any art, or ship with an entirely separate custom-made campaign to demonstrate the capabilities of the reimplemented game engine.

Ducking it wasn't very helpful, what's the 9th circuit? Is it a court of appeals of some kind?

The Court of Appeals for the 9th Circuit is an appeals court that covers basically the entire west coast of the United States within it's jurisdiction. As a result of that, it covers a lot of copyright cases and has a lot of it's own copyright jurisprudence that doesn't automatically apply in other circuits. (Notably, Oracle v. Google would have taken half the time it should have been had it not included patent claims, which moved what should have been a 9th Circuit case into Federal Circuit jurisdiction.)

Thanks for clarifying!

I believe they have rewritten the underlying game engine, but you still need the game for the assets like character and world models, audio, etc.

Fun fact: Morrowind used the Gamebryo engine, which is apparently still in active development in 2021! (Or at least the website is still up)

Thanks for your explanation.

The title is misleading, the creators don't even claim this themselves:

"OpenMW is an open-source game engine that supports playing Morrowind by Bethesda Softworks. You need to own the game for OpenMW to play Morrowind."

Presumably the code is open source, the art isn't?

Thanks for your explanation.