They set the rates. They keep the rights to their own face. They split the money, and the split is on the wall. The person in the picture owns the picture.
Drag the fee. Watch the same job pay two very different ways.
Not a union bolted onto an agency. The agency itself, owned by the people it represents. Profit that used to be a founder's is a dividend that is theirs.
The rights to your image do not transfer at signing and do not linger after you leave. The most valuable asset in the building walks out with the person it belongs to.
The agency negotiates for you, never against you. There is no incentive to book you cheap and often; there is every incentive to book you well.
Every model can see every split, every fee, every deduction. An agency that cannot show a model where their money went is not one they would have built.