Boeing decided that "the customer is always right" when it came to designing the 737 MAX, and "the customer doesn't need to know everything" when there were problems with the aircraft's stability due to the customer demands.
Customers wanted an aircraft with the same training requirements and certification as the 737, with bigger engines and more fuel efficiency.
When that led to aerodynamic issues, they tried to fix it with software instead of hardware.
Boeing did this because Uncle Sam makes it damn near impossible and very expensive to get a new passenger jet certified. So, they chose to retrofit an existing certified plane. This doesn’t alleviate Boeing of responsibility for the deaths, but also implicates Uncle.
I would think certifying a new airframe should be rather hard. The issue could also be they allow expansion of a certification too easily.
The planes fly in US airspace, in US jurisdiction, and by its nature air travel can involve state to state travel. I see no issue in the FAA being able to certify new aircraft, or allow existing ones to me modified under a given certification. I think the issue is also they need to narrow down the scope of modified certification.
The airlines are also not blameless, as they ask for aircraft with modified certifications so they don't have to retrain all their pilots.