I will try and put the question briefly: Contract theory can be seen as a way to legitimize the use of force for the common social good. Contract theory states that one gives up ones natural freedom (jus naturalis) in exchange for being a part of a working society. I’ll keep to Hobbes and use security for an example of a good that comes out of this. Presumably this needs on some level to be a decision you are free to make or not make, and should thus be made, like most contracts by a responsible enough person to be responsible for its implications. This is partially impossible since we are not born thus, but we can opt to depart from society at some stage in stead. But this is no longer possible. There is no place on earth where you are not subject to the laws (force) of some society. How can a contract be valid if there is no choice? (I am by the way speaking somewhat metaphorically here, I do not envision any actual contract, or necessarily an overt decision.)