There are several criteria that a contract needs to meet in order to be binding. Interestingly, being written down and signed is not one of them. Verbal contracts can be binding. The reason for writing them down and signing them is to avoid he-said-she-said type arguments if there is a dispute.
One of the criteria for a legally binding contract is that there has to be an actual "meeting of the minds", i.e. that the two parties have to actually agree on what their obligations under the contract actually are. An obligation to sign some unspecified thing in the future does not meet that criterion because you don't know what you are agreeing to.
(For this reason, a ToS "contract" which allows one party to change the terms unilaterally cannot be binding. You have to be given the opportunity to reject the changes.)
But most people don't know these things, and so they allow themselves to be manipulated by charlatans who do.
Disclaimer: IANAL. But I've signed a lot of contracts, some of which I shouldn't have.