There are still intellectual property laws in place that could protect the original companies. Non-competes stifle innovators at big companies from being able to leave and pursue their own ideas that are in the same domain as the parent company.
I'm not saying the existing solution is complete, either. But banning noncompetition itself is not the right answer. Why do contract noncompetes clauses exist?
What about the small companies and inventors and IP owners? None of these replies I'm seeing have considered the side effects to the world except what would clearly also benefit giant companies.