It's one of those things that are technically true, but doesn't matter as much as the candidate is led to believe.
Negotiating $10k more for a candidate might see them get some percentage of that. But if they place you now, they can move on to another candidate and get a percentage of $100k or more.
Churn is far more important than min-maxing any single candidate.
Yep, recruiters like other middlemen make most money with volume not price/quality. It's not the size of the deal it's the constant flow of deals which makes them money.
For them to pay lots of attention on a single deal it has to be rather large, so that's why head hunters exist for top CxO type personnel, but not really for rank and file positions.
Negotiating $10k more for a candidate might see them get some percentage of that. But if they place you now, they can move on to another candidate and get a percentage of $100k or more.
Churn is far more important than min-maxing any single candidate.