And yet it’s not even that simple. Contracts can be invalidated.

Other countries have fairness doctrines with allow/disallow lists of things that can be included in contracts.

There are other ways.

The US has limits on contracts as well, and courts can and will invalidate clauses or entire contracts for a variety of reasons. Read any of your terms and conditions, there is almost always a clause in there saying that if a clause is found to be unenforceable, you agree that the rest of the contract is still valid. The bar is a lot lower in the states, but it’s still there.

E.g. You can sign a contract to work for less than minimum wage, it will be entirely thrown out in court.