@fizzgig I think that birch bark puts the limit on barkskin, and as far as I know it can be found mostly in quite dangerous areas. So you can only mass produce the essence, not the potion itself. And after you got into enough trouble finding birch bark, it would be quite annoying to fail the easy part of the process (the essence).
As for Eagle's Splendor, all of its ingredients are easy to find, or at least they are not usually in dangerous locations, so it balances out this way.