[emarsden]

There are several good reasons for allowing it. One is that it's difficult for a public inspectorate/regulator to maintain the necessary levels of expertise to assess such complex systems (and increasingly so with technological progress). Furthermore, people working inside the industry have much better access to information about the risks than an outside inspector has.

A second reason is simply costs to the public. In 2019, the interim FAA director Dan Elwell testified to the US Senate after the 737 Max disasters that bringing all delegated oversight back into the FAA would require 10000 extra staff and USD 1.8B in costs. There are fairness/democratic arguments to having the costs borne by the industry (and thus indirectly by the privileged portion of the taxpayers who consume air traffic) rather than by all taxpayers.