Because there is a perverse incentive for performers to lean into scalping: selling out quickly is a mark of success. NPR's "Planet Money" had an episode a while back that covered exactly this.

Not all artists lean into it of course, and it's usually not the actual artists anyway but labels, producers, etc.

In that same episode they covered how LiveNation owns both TicketMaster and many venues themselves, and leverage access to the venues for power in the ticketing market.

It may have been this one but I'm not 100%: https://www.npr.org/sections/money/2013/06/25/195641030/epis...