"by including <Genre defining game NFT> in our game." The part you left out was meant to imply that the "Genre defining game" developer created the NFT's for the purpose of other people integrat them in the game and them getting free publicity,. Therefor rights would have been included.

> Why don’t the mods just put the thing in? Mods don’t need to respect anyone’s NFT ownership.

For the same reasons that people buy games instead of pirating them most likely. "Right click and save" is a fundamental part of the internet yet digital things are still are bought and sold. (edit including DRM type mechanisms if this pattern became a thing that actually mattered money wise).