I would imagine a lot of it has to do with food scarcity. There weren’t trucks and boats and planes to transport food back then. You had to eat what was available in your specific patch of dirt. If there aren’t a lot of food options where you live, and what is available can make you sick, you might start trying to prepare it and eat it in different ways until it stops making you sick.

For most people, rhubarb is one of hundreds of options of things to buy from a grocery store. To our ancient ancestors, it may have been one of a small handful of things that grew where they lived, and therefore a necessity to figure out how to eat it.