People need to eat. It’s just simply part of being within the ecosystem on Earth. When we voyage off Earth, as in an airplane ride or a visit to the International Space Station, then we bring our foods with us. And any waste gets carried along to re-enter into the Earth’s system when we land.
Not so if we want to put a colony on the Moon. On the Moon there’s no ecosystem. Nor is there any medium like flowing water or blowing atmosphere with which to transfer chemicals and energy from one life form to another. On the Moon or any other non-Earth location, people must bring along their own, artificial system.
And if we decide that we don’t like the system on Earth then we can even use the same concept here. Just as Seven-Eleven Japan has decided to do. Which may be a boon to space enthusiasts. But what does it say about the Earth ecosystem?