This is how it might work: The hospital or group has (privately) earmarked up to $50K to bring in a badly needed doc, yet they're only advertising $25K. They're happy to give $25K to a recruiter and $25K to the doc. But you find them without a recruiter because the $25K in some ad you saw, caught your interest. There's no recruiter to give the extra $25K to. But it's potentially available to you, if you ask for it, i.e. it's negotiable. If you don't ask for it, and they can get you just as easily with a $25K carrot, then that's what they put in your contract.
TLDR: Once a recruiter can tie you to them, they can leverage a chunk of money as their fee, from money that otherwise might have been available to you.