Because employment is more than wages, and some of the constraints on these employees working arrangements are going to be bound up in the terms of the contract

Because employment is more than wages, and some of the constraints on these employees working arrangements are going to be bound up in the terms of the contract their employer has made with Google. If their employer would like to offer them changes in working conditions or pay that, in order to be feasible or economical, they would need to change the terms of their Google contract, they can't do that without Google's involvement. It would seem far more efficient to have Google be in the room involved in the negotiations and bound by the arbitration, rather than have the contracting company have to go to Google and suggest something they might like to change, get tacit approval that that might be up for renegotiation, then go to their employees and make an offer based on that assumption, and if the employees accept it, hope that Google honors the tacit agreement they got rather than laughing and telling them how they now have them over a barrel and further tightening their contract.