The idea that there are no resources we know of in space which are not more easily accessed on earth is just outright untrue, or at least is only true in a narrow sense. My example here would be Helium-3, the ideal fuel for fusion (a difficult choice due to high fusion temperatures, but it has the advantage of not kicking off neutron radiation in the process the way something like Deuterium-Tritium fusion would). Earth contains ~10-50,000 tonnes of feasibly accessible Helium-3, and if we were to move over to fusion power at a large scale at our current rate of power consumption, we would consume that amount of fuel in a matter of years, likely less than a decade. By contrast, the moon contains orders of magnitude more Helium-3 in its regolith, somewhere in the ballpark of 600,000-1,000,000 tonnes, a sufficient quantity to last over a century in the same usage conditions as outlined for Earth. Additionally, both of these sources pale in comparison to the amount available in Sol’s gas giants.

The caveat here is, of course, that it’s unlikely we would switch to fusion entirely in the first place, and that accessing that helium-3 at scale is not easy, no matter where it comes from (though doing so at scale is likely easier on the Moon than it is on Earth). It also ignores ideas like degrowth, energy efficiency improvements, dealing with the drawbacks of alternative fusion fuels, etc. I think, however, that it remains illustrative of the larger point: there are compelling reasons to go to space, even from a raw materials perspective alone.