People can work it out at great expense to developers and at the risk of destroying whatever a game is stylistically.
Again, each game will have its own ways of presenting, combining and displaying objects. A game with highly detailed human-like motion will be radically different from a cartoonish game. A game designed with large textures in mind will not work well with small textures. A game with complex physics will be completely different from a simpler game in how it represents, handles and combines objects. And so on, and so forth, ad infinitum.
Brushing this aside betrays how little you understand of this.
By way if example: websites are completely open systems. You can go in and see all the "assets and objects and items". Websites are also manny, many orders of magnitude less complex than a computer game.
Go ahead, copy a website's CSS and paste it into another. Or copy a chunk of a website's HTML and paste into another. Doesn't work, does it? You can reflect on why that is.
On top of all of that, why would, say, developers of Elden Ring want to allow you to bring in assets from Horizon Zero Dawn or Binding of Isaac? Those are completely different games with completely different stylistic choices.
> I understand why AAA games are so boring nowadays outside of a couple studios.
Ah yes, they are boring because you can't use skins from Fortnite in Shadow of Mordor. That's the reason.