Because artists need an at least semi-competent vis-a-vis, AKA audience. This is the main proposition for entering a stage to begin with. And chances are that the most economical potent ticket buyers are not in this group and probably also not the most enthusiastic about the act. So there's a natural incentive to aim not for the highest cap, but for a somewhat realistic medium. I.e., "the prices people are willing to pay" are probably not the prices artists are willing to perform at.

(Edit: there's a reason for opera houses providing cheap standing room for enthusiasts – it keeps the art alive.)