It would seem that the U.S. has a pretty poor constitution if it can be superceded by contract law. Isn't the whole point of a country's constitution that it stands above all "lesser" principles and laws.
The US constitution was written with the mindset that it should be a meta-law to limit the power of an oppressive government, so it only says things like "Congress shall make no law (...) abridging the freedom of speech" and doesn't really deal with people at all except in the context of government action like giving you a fair trial. Everything else is just law, it's illegal to murder you not unconstitutional. So if Congress wants to do something about this, they should pass a law. The flip side of this is that despite being called the Bill of Rights there are actually no rights granted. If the government did absolutely nothing, didn't hold any trials because it didn't bother to prosecute any criminals that would not violate the constitution. I think the closest thing you get to a "right" in the constitution is the 13th that says: "Neither slavery nor involuntary servitude (..) shall exist within the United States" but the wording is more to say that such contracts are null and void so the usual laws apply.
You could write an entirely different constitution centering on people, saying basic things like that people have a right to freedom so that slavery and involuntary servitude is outlaws is merely a consequence. That everyone is entitled to equal protection by the law so for example selective enforcement would be unconstitutional. But that would be an entirely different way of thinking about what the role and purpose of what the constitution should be. It's probably more in line with the way we think about civil rights today, but it's a very different world today than in the 1700s. Even basic things we take for granted today like elementary public education didn't exist, you had some free schools run by churches but it was neither universal nor compulsory. The government was mostly the law, the rest you had to take care of on your own.