Well, there's a difference between knowing how to weld and being a professional welder.
My dad runs a farm and he knows how to weld, and he's usually done it himself. But lately the kind of welding he needs done is beyond his skill. That's because he hasn't gotten any training for it - he learned the basics and then worked on that by doing it.
So nowadays he has to hire a welder who runs his own mobile welding business and hires 3 other welders. He does those tougher, more complicated welding jobs. He has some certifications and he also has a portfolio of his work.
But the difference between my dad's capacity to weld and this professional's capacity weld is that, well, he's a professional. By that I mean he understands things like how different metals interact together and how to watch out for things like water traps that will cause the metal to rust and basic engineering things like stuff that will likely fall apart. My dad wouldn't know any of that stuff regarding welding because he's an amateur and he saves money by doing what he can. But the professional welder knowing intricate and detailed things about that craft is why he gets to charge $80 an hour for his services.
So on one hand you're right in that certifications and degrees are being used as gatekeepers and yardsticks for competency. But what my point is is that more technical skills require that kind of certification more than academic degrees.
And the reason why is because those technical skills are more science based than academics are. So technical skills require more education to develop those skills but also demand more pay for those skills.
I guess xkcd puts it best:
xkcd: Impostor