There’s stuff like Curseforge, but it’s only for some games, mostly Minecraft. The problem, if someone considers it a problem, is really that communities for games generally centralize around one site for their mods for the most part, and Nexus has garnered a lot of trust and therefore has more pull/inertia for communities working those things out.
As for Github, I believe the vast majority of mods have Github pages, but Github itself doesn’t really have a UI suited for mod downloaders, and no real incentive to implement one. So sites like Nexus and Curseforge are still a necessity.
ryven