> Because artists don't always want to extract the maximum money possible from their fanbase?

I think that's both true and not. The larger truth is that trying to maximize the extraction during a single ticket sale is incredibly short-sighted of an artist. Having fans attend shows is a very effective way to grow your fan base and your brand, and that brings so much more lifetime value for an artist than you'd ever get from a single ticket sale (except for maybe on your retirement tour --and even then).