I think there's still a lot of inconsistency and judgement that happens for gauging whether someone is junior and for what skills they might have. Many software engineers don't have CS degrees at all and even more have ones from different countries.
I am unable to find concrete numbers on this, but I would bet that the majority of employed engineers in the US do not carry a professional engineering certification.
I don't argue with your point regarding how many engineers have their PE. I don't have mine.
My point is that accreditation boards and trade unions can help set standard minimums of what an engineer (or trades person) will know going into a job. Of course employers can ignore them, but in general those standards help bring consistent expectations to a given trade.
I am unable to find concrete numbers on this, but I would bet that the majority of employed engineers in the US do not carry a professional engineering certification.