I have an associate of applied arts degree in engineering and a bachelor's in operation management. I was first hired by Lockheed Shipbuilding in 1965 as an engineering technician. Since then, I have worked as an engineer in various capacities. I have spent the last 10 years working for the Coast Guard as a civilian general engineer. No one, to the best of my knowledge, has questioned my competence to do the job.
Competence has little to do with passing tests or taking exams.
It is based on your ability to comprehend the material and concepts, your experience, and your ability to apply those things to the real world, i.e., design things that work. Unfortunately, a lot of engineers think they are designers but have very little ability to design something that works, even though they can spout the basic concepts and underlying principles all day long. For these people, certification would not have any meaning. It would only be deluding the public. Any group or sector of society tries to find ways to differentiate itself.