The missing link here is that the U.S. government policymaking process is very much tied to citizen sentiment. Politicians who attack popular things can find themselves out of power.

As long as most people are OK with Amazon, there will not be much public demand for government to make new policy to constrain them.

Policymaking is the key process because it’s not illegal in the U.S. to outcompete other companies, regardless of how big or small they are.