The real reason is that every single aircraft development program in recent history has been late and over budget, and now aircraft development is something that costs an eyewatering billions of dollars, if not in the tens of billions of dollars at this point. Bombardier sold, Embraer tried to, and Japan, Russia, and China have all mostly sputtered on despite having well financed government backing.

To put this in perspective, 787 development cost $32B. 737MAX is expected to cost past $18B. You can't bootstrap a startup if it takes tens of billions of dollars to get a plane out of the factory.