Except, you know slavery is illegal even if both parties are informed adults who could agree to such a contract. Same for indentured servitude, or selling organs. And these are just the most obvious examples.

There is a line that needs to be drawn somewhere in terms of what can and cannot be allowed in contracts, the question is where to draw it. The other obvious point is whether both parties are acting in good faith, which is why there are laws about fraud on the books (even if they aren't always enforced for some financial industries).