Sometimes monopolies exist because it's extremely hard to succeed even if no one is stopping you

The concept is called "high barrier of entry"

I'm sure airplane manufacturing falls under the highest barrier to entry besides maybe space exploration

Boeing merged with one of their competitors, and now outsource to subcontractors; both of these suggest the barriers, high though they are, are not insurmountable.

Also, the FAA could ask people from Lockheed to mark Boeing's homework.

After a bit of Googling, looks like the FAA do also get Airbus to do this to Boeing, despite Airbus being famously not American: https://www.reuters.com/business/aerospace-defense/airbus-wi...