I think brokers are still in the property game because landlords typically aren't sufficiently informed to know that they can avoid paying a broker, or can shop around for a cheaper one.

Brokers have done a good job of telling landlords that if they don't use a broker, or use a cheap one, that they'll get bad tenants and that'll cost them a lot in the long term. Good broker = good tenants = worth getting 8.4% less, because you'll lose more than 8.4% when a bad tenant burns the place down....

I'm unconvinced that the above is true, but it's certainly the message brokers (fairly successfully) give landlords.