They are offering a service which might include accommodation or driver, security, marketing, answering calls, dealing with problem clients, dealing with cash. They take the risk on all that so the girl knows she will get paid a fixed amount for the clients she sees. If its quiet the agency loses money rather than the girl who still makes some money.
Agencies give the girl knowledge of the market, where to work, what hours, what to price at, dodgy clients, etc
Also many of us lack the motivation or ability to do everything ourselves. I've worked both employed and self employed, self employed I can earn more per hour than employed but I've usually earned less because I don't have a sales team incentivised to bring me work.
Most girls aren't stupid - or no more stupid than the average person - they go with an agency because of the benefits they see from it are more than the commission taken by the agency.