After all, the resources outside the wall and the resources inside the wall are the same.
Some plants are common, some plants are uncommon. Everyone figured, as long as they took away the uncommon plants, that was good enough. Who cares whether those plants are truly precious or not? At least they're relatively rare.
As long as the plants are rare, then taking those plants is obviously much better than taking the common ones. After all, if those common plants run short, you can just risk going outside the wall and pick some of those ordinary plants directly.
But for those scarce plants, if you want to risk going outside the wall to get them, you'll have to search carefully, and keep searching for a long time before you can find them.
