Could it be that the agencies insist that their girls work exclusively for them and never independently all the time they're on the books?
Some might do this but the idea of course is the Agency works for the WG, she is self employed and a free agent. In exchange for them getting her her punters and advertising her she agrees to give them a set cut of her fees. Obviously it doesnt always work out like this as i have spoken to ex-Agency WGs who were threatened by the Agency owner when they said they wanted to go Indie or work at a Parlour as well.
Just wanting to turn up to suck and fuck is why some WGs work for Agencies, their punters are laid on and they havent got to attract them themselves or communicate with them before the punt. Same reason some choose to work for Parlours of course, difference is a Parlour also always provides the premises, a WGs biggest outgoing, not all Agencies by a long chalk do that.