It is very convenient for venue owners to let TM handle ticketing for the events. TM handles more than just sports and concert tickets. Since so many forms of live entertainment use TM, the company can bring in a lot of events to fill in off-season time. So an arena can host basketball & concerts on the weekend, then fill the weekdays with minor things like dirt bike races.
This situation repeats in practically every major metro. The massive arenas are probably owned by the city, or maybe the owner of the team that plays in them.
Which goes back into my argument, they own some venues. But not all of them, and certainly not the major arenas that mega artists play in.
An interesting thing about AXS tickets is that they are presented using a smartphone app and are only valid for something like 60s before they need to be refreshed. This presumably makes tickets harder to resell outside of AXS's system.