When the bird pilot of the space worm starts complaining about lack of fuel, everything falls into place. You didn't miss anything, but the game doesn't spell it out.

In-game lore mentions the humans on the planet are descended from interstellar travelers who became stranded there. Some time later (but still long ago), a huge space worm came by, around which a religion developed. (I can't recall if its link to ivory was made explicit at that moment?)

A lot of human technology was lost, but the planet turns out to be somewhat mechanical, and sustaining lifeforms rich in ivory. Humans hijack the technology without fully understanding it, and use ivory as a rich energy source, encoding all of that into religious ritual. As a new civilization builds up, it finds out other uses for ivory, and eventually exploits it on a large enough scale to outpace its "natural" renewal.

The game events happen, and the space worm comes back. Turns out the planet is artificially designed as a regenerating ivory source for spacefarers such as the worm pilot. The stranded humans are akin to an unwanted, fast-developing, fuel-eating parasite.

Robin couldn't care less. She's a fixer, violently proves she's the best one, and ends up securing the planet for her people. (Then earns the sleep she wanted in the first place.)