Well Vagabond 47, one 'demonstrated' reason why, is that there are pilots who ACTIVELY search for brokers to work through.
I can name at least half a dozen of my colleagues who elected to join an airline on the EXPRESS condition that they did it through a 'friendly' broker and NOT directly.
I know the reasons, I just think that a balanced argument should be seen that at least some of the agencies are actually helpful to the pilots themselves in carrying out their own choice.
So not only the brokers are out to line their own pockets, but also the airlines (avoiding costs) and pilots (avoiding paying to many 'contributions'). When you get that scenario, the agencies are here to stay.
Learn to live with it.