Well, nothing - in theory. You can, of course, license your code in such a way that you can at least send a cease and desist to a cloner, but that's really only useful in jurisdictions which would enforce it.

But for the example of shapez, there's a lot of added value that's not in the source code. The creator is active in the shapez discord, and there are also paid DLCs on Steam which aren't open source. So there are at least some ways to monetize and make your community loyal that are still compatible with open source.