I love sandwiches. Clearly, they are the world’s 3rd best food.1
At a sandwich shop or deli it only takes couple of minutes to make a pretty good one, and yet, I make pizza and cheeseburgers all the time, while I almost never make a sandwich more complicated than grilled cheese.
That’s because the type of sandwich I really like is complicated and takes a lot of ingredients: (good, fresh) bread, turkey, ham, bacon, cheese, lettuce, spinach, tomatoes, avocado, green peppers, olives, peppers, mayo, and mustard. No one in my family wants all that crap on their sandwich, so there are no economies of scale to be had. Plus most of those ingredients come in large-enough quanities that plenty would go to waste before I could eat it all (I’m looking at you, avocado, and your 30-min half life). So sadly, the kind of sandwich I like only makes sense at restaurant scale, where the ingredients are already prepped and co-located.2
Space resources are kind of like that: a deconstructed sandwich where the delicious ingredients are spread all over cislunar space and the broader solar system. We need water (people, feedstock); regolith (radiation shielding, feedstock); oxygen and nitrogen (people, propellant); hydrogen, hydrocarbons, carbon, and hydrides (people, energy, propellant); sunlight (energy); reduced metals (construction); silicon and silicates (glass, semiconductors); and fancy things like plants, animals, and (I suspect) Earth-level gravity.
Low Earth orbit has a protective magnetic field, intermittent sunlight, and a good view of Earth—but not much else. Go higher and you get more sun but more radiation. The Moon has regolith (a melange of metal oxides), hopefully useful water, and a little gravity. Near-Earth objects have regolith, reduced metals, carbon, nitrogen, and water—but have to be moved around. Mars probably has most any element we’d want, more water, a meager atmosphere, and more gravity—but it’s also really far away in ΔV and time (even if you’re light). The main belt has…etc., etc.
So space has the raw materials we need for a delicious space settlment sandwich, but it’s all spread out, far away in transit time and/or velocity, and not yet ready-to-eat.
Hopefully you like freeze-dried dinners.