This isn't so. Most employment is at-will. This works fine, because lots of laws govern what employers may do to employees, and anyway slavery is illegal so any employee may leave at any time. 95% of any employment contract is various attempts to get around those legal protections for the employee.

This is an especially pernicious case of the normal Stockholm Syndrome we see whenever the special status of corporations is challenged. No firm is 100% robots yet. If we withdraw our consent, they have to raise wages and improve working conditions until we relent.

The law only protects you from actual crimes. There are hundreds of other small provisions which have to be negotiated as part of a contract to be enforceable from a legal perspective.

This is especially true for freelancers, who don't have a single employer and are instead usually paid for deliverables as specified in a contract. Being able to quit anytime doesn't help if you've already spent weeks or months working on a project and never get paid because the company decides on a whim that they don't need your deliverable anymore.