To my understanding, GrapheneOS currently coming with a plain black background isn't just due to nobody bothering to make a custom one. The Material You theme is based on the used wallpaper. This means that apps might be able to indirectly fingerprint you based on your background. Giving everyone a plain black one is probably one of the best ways to mitigate this at least.

If in the future the devs provide a fix for this, maybe there will be some standard wallpapers. However, I can also see an argument for staying with only plain black as a deliberate choice in favor of minimalism.

As far as I understand, the fingerprinting can only happen if the custom accent color that are based on your wallpaper are used, not with the standard ones.