Because you should think of contracts as underspecified interfaces between two parties. If I'm actually unhappy with you and want out, I can perform my side of

Because you should think of contracts as underspecified interfaces between two parties. If I'm actually unhappy with you and want out, I can perform my side of the deal adversarially - technically and legally in line with the contract, but otherwise not exactly what you expected. So you have some incentive to keep me happy and renegotiate up the rate voluntarily, since otherwise I will act adversarially on all the things that are not explicitly specified in the contract. This can work depending on the extent to which the contract is underspecified. You will note, for instance, that the hotel cut off room-service access to this restaurant. Clearly they didn't specify room-service availability in the contract, and so on.