It’s a feature of the Co-op system known as tether. The game doesn’t load the full map and all features all at once, but instead it runs only the area where the host is. That’s why players can’t wander too far away from eah other. There’s no way around this in Co-op mode.
However, there is an alternative. You can host your own private server. If your PC has the hardware you don’t even need a separate machine to run the server. That way, the whole world is your sandbox at the same time, and being the server owner you can shoose who gets to join.
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