The Fed Board of Governors (the body that makes monetary policy) is an agency of the federal government. That means that yes, it is centralized, but it also means that it operates under a public, transparent, and democratically changeable legal structure. If Americans don't like the Fed, they can vote for a president and members of Congress who will change it.

Bitcoin has become centered on the few largest mining operations. Libra is centered on Facebook. Who elected them? How do currency holders influence them? There are no defined ways to do so.