Because knowledge is currency. It opens doors to privilege and status in society. It also ensures incompetent people are not put into positions where they can do harm.
Sure, but why would you hire an unlicensed electrician, or surgeon, or car mechanic, or builder, or elevator mechanic, or really anything that matters?
The only areas where this point becomes moot are in areas where certifications already are not an issue, i.e. in jobs that almost anyone can do.
Also, I’m fairly certain car mechanics are not licensed where I live. Sometimes licenses are just frivolous, especially in modern day society.
You don't need a licensing system, you just need a reputation system. Like how bonds have a rating system; nobody's stopping you buying a junk bond, but the system makes it clear to you that it's a got a high probability of default.
If it is not a matter of life and death, there's no possibility it will happen, because of regulations.
Anyway:
More than 250,000 people in the US die every year because of medical mistakes, making it the third-leading cause of death
10 percent of all U.S. deaths are now due to medical errors
can I sell it?
Can I buy grocery with knowledge?
Knowledge is a tool!
And it's not even universally true, if you know American English it is perfectly useless in rural China or Japan or Central Africa.
EDIT: I'll add another example that won't upset the American audience.
Numbers in French.
We are used to the decimal system, it won't work in France, they count numbers using the vigesimal system.
So 84, 80 + 4 is quatre-vingt-quatre, 4 x 20 + 4.
My way of counting numbers, which is a basic requirements for kids aged 5, is completely useless in France, even though France is a close Neighbour of my country and we dealt with each other since the dawn of history.
How are you telling whether you are successful at that? Even if you do not care about the personal individual achievement level (or whatever) of the students, you still need to be able to measure to understand where you are successfully teaching, and not, so that you can change/improve/etc teaching.
As the comment you replied to said, you can't just wish these problems away, and they are not easy things.
The overall thing is not a problem, they are systems. They can't be "solved" through simplistic answers
Some things to understand about teaching. You must always teach at the middle kid in terms of ability and intelligence. By default this already means that some kids will be lost and some kids will be bored. This is made worse by conflating age with competence. Additionally, teachers have no understanding of what the kids are going through at home. Say you have a kid that never does homework? Is that the teacher's fault? Is it because the kid is lazy and just plays fortnight at home? Or is it because the parent's only job is night shift and the kids is a de facto parent watching two other kids? Or is it because the parent has a substance abuse problem and the kid hides out at playgrounds until late at night after everyone is passed out and it is safe to come "home"? Statistically, kids with problems at home also tend to be lower on competence scales. The real problem here is social help for the parents but we don't have the political will for this. Do you have any idea how often a teacher has had a student's parent come to a conference to discuss concerns about the kid falling behind just to be told that "It is YOUR job to teach my kid, not mine!" Tell me how testing teachers fixes that? And these are the same teachers that must buy paper/pens/supplies with the other salary because we ration school supplies.
We have some similar problems at the collegiate level. I worked full time while carrying 12+ credits paying my own way through college. I had to cut corners and ration my time. This meant lower grades for some classes but luckily I have the aptitude to get away with it. We also are sending kids to college that shouldn't be there. They don't have a real desire for a professional career outside of something like Social Media Manager. Of course they are going to cheat and use all the tools they have at their disposal having grown up digital. They aren't interested in the subject matter they just want to check the boxes and get through it. There is an issue here that needs solved at the institution level that kids will always be better at tech than the teachers but that is silly to lay at the feet of the teachers. In the end they are trying to lay a foundation of knowledge but the students have to care. Most college classes don't take attendance, is that the teacher's fault too?
Having the best software engineers doesn't mean anyone will use the product. Having the best doctors doesn't mean patients will do what they are told. Having the best trainers doesn't mean people will workout on their own. Having the best therapists doesn't mean anyone will use the techniques suggested in their daily lives.