Ticketmaster is owned by Live Nation which owns at least 338 major concert venues [1]. Their market power in the venue business allows them to force artists to use Ticketmaster for ticket sales. The artists don't mind though, as they can tell their fans they have no other choice but to use Ticketmaster. Ticketmaster absorbs all of the reputational stink and the artists likely earn more money than they otherwise would have if they were forced to sell tickets at the low prices their fans want.
Except that they don't absorb all of the reputational stink because "Live Nation owns at least 338 major concert venues" is clearly a BS excuse when there are more than 10,000 concert venues in the US, and then the fans still blame the artists for using Ticketmaster.
[1] https://en.wikipedia.org/wiki/Live_Nation_Entertainment