Ticketmaster is part of LiveNation. LiveNation has exclusive deals with almost all of the large venues (stadiums etc.) saying "all shows here must sell tickets via ticketmaster", and deals with many of the artists themselves saying "tickets to my shows anywhere will be sold via ticketmaster".

So any potential competitor trying to undercut them would have to sign new artists and not have them perform in any of the established big venues. They have a functional monopoly, it's literally impossible for a competitor to sell tickets to a concert at a given stadium or for a given artist for cheaper, because TicketMaster/LiveNation have contracts that make it illegal.

That's the problem, they can charge whatever they want because they have a monopoly where it's impossible for someone to undercut on the same product, because they're the only ones that are allowed to offer that product.