There is
https://opengameart.org/ . Major limitation to this idea is that for a decent looking game the assets need to be consistent. Not only in terms of art style, but also detail level and technical constraints. For example if you want a modular building parts some random assets created by different artists without targeting a specific game requirements will probably not be compatible with each other or building system in game. Not only 3d model can be too low quality, having too high quality can also be a problem. You don't want some unimportant background prop to consume more triangles than main player character. It may look better to have less and worse quality but consistent assets, than more mixed quality assets.
It might be slightly simpler if you assume realistic 3d art style. But even then what's considered realistic 3d game art has significantly changed over the years.
Maybe this would work better if the repository served not only dumb file storage but tried to establish set of target requirements (updated once every few years) to improve the compatibility assets. Something like opengame-3drealistic-2020 or opengame-2dtopdown-pixelart-v3. Where each category means very specific rules and maybe some reference pieces meant to ensure compatibility. Downside is that it significantly raises the entry barrier and reduces amount of artists willing to make something which fits very specific requirements, instead of making whatever they want and then sharing their work. I am also somewhat concerned about loosing creativity.