I think I have an answer. The focus isn’t the separation of the waters (chaos above, chaos below)… but about the “vault.”
Some bibles say “firmament,” and some say “vault,” but the image is this space created between the waters that separates the waters above from the waters below. And it’s inside this separation where God separates water from land. Inside this special place.
I often think about working against the curses of Genesis 3 & 4, and how we’re meant to shine as lights in the darkness. A part of our Christian prayer is “on earth, as it is in heaven.” And it’s a prayer to create a protected space in this life where God can be seen clearly…
This protected space is like a vault that holds back the waters of chaos & darkness. It’s space where water is gathered away from land, which represents humanity. And life flourishes on that land.
When we pray “on earth, as it is in heaven,” we’re praying for the firmament.
So why does God not say “it is good?”
Perhaps it’s because it isn’t good *yet.* It represents the good we MUST DO later, after the curse of sin. It WILL be good.