The technology already exists. Fundamentally (damn laws of thermodynamics) it is also a energy negative process, and hence expensive. It is far cheaper to just frack and burn fossil fuels (where that energy expense was paid however many millions of years ago by someone else), than it is to expend the energy to reverse the process - even if they were getting nearly free energy.
I don't think that anyone is trying to make the claim that SpaceX will use the Sabatier process to fuel Starships here on Earth right now routinely. As you say, gas is cheaper to buy here on Earth. But they still must have the technology mastered before they fly away from the low Earth orbit.
This isn’t something unknown or theoretical.
It is up to the controllers of incentive structures to change the fundamentals of commerce/ecology.
Not saying all human endeavor needs to have a healthy profit margin. SpaceX is a business however and besides doing it for the lulz, or getting a government contract, it doesn’t make much sense to try to spin up colonies or the like there. Everyone will starve and/or their US backers will go bankrupt pretty quick.
