That's not the spirit of the quote, though it's not entirely your fault. It's better reflected in the original, attributed to Aristotle: "Those who can, do; those who understand, teach."

There are a whole host of reasons someone may not be able to "do". Maybe they used to effectively "do", but got hurt, or just got too old. It could be the simple recognition that teaching is the "do", because of your deep understanding and ability to communicate and inspire.

It recognizes a conscious choice not to stubbornly attempt or continue to "do", but to instead take a back seat and direct your energy into others who will "do" more effectively than you ever could. It's about wisdom, self-awareness, and taking the long view. You are readying the next generation of do-ers, passing the torch, perhaps multiplying your efforts and extending your life's impact further into the future.

I think it comes from jobs where a physical limitation prevents people from “doing,” like retired athletes or really experienced surgeons who develop a tremor. Or even just people who no longer have the physical strength to do manual labor, but have a repertoire of techniques that will help others do it more effectively.

I agree that the quote doesn’t apply to teachers in general.

The US isn’t especially poorly educated on average.

The US is so unequally educated for many reasons, but the biggest one is the cost, which is mostly due to (as always) Reagan. He defunded K-12 education and public universities. When universities raised their prices in response, the government, which had been offering some need based student loans regardless of major since the sixties, greatly expanded the program, partially privatizing it. In order to make loans to very young people without immediate employment prospects a less risky investment, the loans were made unable to be discharged in bankruptcy. The increase in available loans allowed universities to further increase costs without risking pricing their applicant pools out. That became a vicious cycle and the tuitions and loans became so bloated that many graduates start with six figures in debt. It’s not a good deal for a lot of students, and it’s a worse deal, the less stable the finances of your family, unfortunately.