No, it’s similar to how film is expensive now; the demand for the product decreased to near non-existence so the few people who still make it have to demand a high cost to be able to keep making it.

They’re still paying artists (presumably unionized too, from what I know of the industry, which isn’t much). They’re just not paying the cost it would take to get the ancient technology back into manufacturing, and train new artists to use the tech that no one is trained to use anymore. (It’s more than just watercolor. I’m sure the paint is special, as well as the material it’s painted on, and the tools that help them use it all.)