Consistent educational standards leads to a common shared understanding of what knowledge and training your engineering colleagues have. This also makes it easier right off the bat for an employer to know what a "junior" engineer is - and leads to more consistent experience levels afterwards.
And you didn't quite mix it up, a degree from an ABET accredited school is required to get your professional engineering certification, which is often required in cases where engineering designs carry legal liability.
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.
