[Tsuwabuki]

Not everyone who can speak English, even natively, can teach English. It's true that some level of learning can be accomplished through conversation with any speaker of English, but for older students there is a point where mere internalisation ceases to work. For adults, especially, the need for grammar structure to be explained in the native language of the student is absolutely critical. By the time we enter adulthood, generally, our worldview is so dependent on our language that internalisation simply no longer works. This is why children have such an easier time with language acquisition; they have yet to see the world primarily or entirely via one language. The ability to actually instruct individuals on the parts of speech and linguistic structure of English takes years of study. A college degree at an English speaking university at least gives you the tools to recognise mistakes, and a degree in English specifically gives you the ability to know why they are mistakes and how to correct them. Adding on education courses allows you to instruct others as to why they are mistakes and how to correct them.

Native level fluency alone does not, and will never, a teacher make.