Because the artists are all in favour of it. Ticketmaster et al are in the business of reputation laundering. If the artists sold tickets directly at market clearing prices then fans would be livid at the level of greed on display.
By using Ticketmaster as an intermediary, artists are able to put artificial below-market prices on the tickets but then sell them at inflated prices, while deflecting fans' anger to the middle man. Ticketmaster, in turn, pays the artists a kickback and everybody wins (except the fans).
