A man who sells leeches has the benefit of ignorance. How many people understand what exactly leeches are and what they do? Not many, I’d wager, and I’d count many a medic among those numbers, which makes it all the easier for them to sell leeches as a miracle cure with the conviction necessary to make a profit. The same can’t be said for cabbages. There’s no mystery. They taste shit even in a stew and I’ve yet to find a meat they compliment.

[That said, leeches have fallen out of regular use in his world. They've developed more effective methods of dealing with medical problems that might have, at an earlier point of time, been addressed with the application of leeches.]