Because Livenation/Ticketmaster have a hand in almost every step between the artist and their fans.
They either have exclusive deals already in place, or they even outright own parts of it.
The artist’s manager has contracts with them, they have contracts for concert promotion, and overall the vast majority of venues have exclusive ticketing contracts to only use Ticketmaster.
There are very few large venues that will even give artists the option of using a different ticketing service.