@Kalamel
Because relying on a single resource source, one with limited lifespan and risk of dying randomly at that, is quite risky and not bound to be self sufficient in the long run if he disappears or the population gets too big.
Additionally, he may know a plant on sight, but unless he knows about the plant, he cannot recreate it. On top of that, he can't create metal or other stuff with his skills.
And sure, you can import it via trade, but it is always infinitely more preferable to have it on your own lands