Gamedev here. Each engine and each game will have vastly different asset pipelines, you can't just copy something across and it just works. Models, textures, shaders, animations, etc usually go through several stages of internal tools before making it into the game. Of course, games with user generated content will make tools available to create things for that particular game, but you can take that and use it somewhere else.

And that's just the technical side, design and art would be horrified if you have no control on what can be added to a game.