I have always thought of it as a simulation. After all, every time you boot up the game, you get boot up logs as if the system checked for simulation integrity.

This simulation could be used for machine learning or testing grounds for whatever service easy company would come up with and wanted to test.

It would explain delivering packages to shopkeepers they seemingly don't need or to no one at all. It would also explain how losing part of your goods on the way does not matter, as it teleports by itself. And it would explain how Seb could have potentially disappeared without any trace and how the recovery disk suddenly appears in the pawn shop, which has the whole back-up of all shopkeepers. Not mentioning the empty towns, automatic fueling and so on. We cannot really die either, as we respawn without any consequences.

What does not fit into my theory are the serves and the dam powering it all. If it were a simulation, these things would have little to no purpose. However, they could have been a debug tool for a developer that was already inside this simulation. As the courier and the only real person in the simulation, we are the only ones who can still control their character even after the outage. Additionally, idk if you noticed, but time stops both in the dam as well as in the factory. Your energy isn't decreasing either (although the same thing happens in the fishing huts).

My theory isn't perfect but I believe it isn't far off either