It’s network effects.

Because a buyer’s agent won’t show a house that isn’t listed by another member of the cartel. So if you want most buyers to see your property, you have to use the cartel.

And you don’t get to view a cartel house unless escorted by a buyer’s agent. Also, the seller’s agent will discourage the seller from accepting offers from buyers not represented by the cartel. So if you want to see most properties, and have your bit taken seriously, you again have to use the cartel.