> Space and planets we can visit has absolutely none of these things.
Because of that, you'll be carrying with you sources of better breathable air, fresher water, and food that's much less likely to kill you or escape you. At least early on, before the market pressure reduces aforementioned consumables to lowest possible quality still able to sustain some degree of life.
While this is true, it doesn’t address sustainability of a settlement. Sure, many pioneers carried some rations with them and known good water, but at certain point, you will need to obtain resources from your environment.
The most salient problem here, however, is that the quality of each resource is of little concern if I end up running out of it for whatever reason and there is only a limited exit strategy.
Earth is an island among a vast amount of inhospitable nothingness sprinkled with the occasional dead rock or gargantuan ball of toxic gas orbiting a fireball blasting out photons that degrade our materials and the DNA in our cells. Early earth-bound pioneers of “new” lands could be confident that they could at least breath the air, but this environment is confirmed only to exist on planet earth for now.
Because of that, you'll be carrying with you sources of better breathable air, fresher water, and food that's much less likely to kill you or escape you. At least early on, before the market pressure reduces aforementioned consumables to lowest possible quality still able to sustain some degree of life.