1. The enjoyment of attendees can be partly dependent on the enthusiasm of other attendees. Consider the extremes: an audience entirely comprised of people who hardly know the artist, but can afford it Vs an audience comprised of only die-hard fans, irrespective of their ability to afford a ticket. The first has a flat atmosphere, the second, a special one.
2. A simple desire by the artist to make their show available to a cross-section of society (this could be viewed as altruistic, since most other products/services don't usually offer the same thing at a lower price just to help those who can't afford it).
A possible solution to problem 1: If the goal is to ensure that only die-hard fans get the tickets, why not use their spotify/apple music/other histories to work out who's really a fan of that artist? It should be trivial to get a very high degree of accuracy, and would be costly for scalpers to imitate (they'd need a subscription and to listen to random music years in advance).