Most of the agent-bearing talent are performers (recorded or not)-actors, directors, speakers, musicians, models, athletes- who negotiate and set compensation ahead of time. These circumstances are what lend themselves to an agent middle-man.

If you are a lawyer, architect, accountant, engineer-a professional billing to the client on an hourly basis-an agent doesn't want to work for you. Cf below-the-line Hollywood professionals, like colorists, sound engineers, cinematographers, these guys don't get agents.