There are, to put it bluntly, a lot of ESL textbooks for adult learners that are, to varying degrees, crap. There are many and varied reasons why these books are awful. Many of them are weighed down under more than one layer of language guano. You're waist-deep before you even get started, digging through the stink to find something useful.
I can count on one hand the number of English teachers I've met in the ten years since I first came to Korea who were actually certified teachers back in their home country. If the proportion topped 2%, I'd be...
It's pretty often the case that Teaching English in Korea involves very little teaching and not a whole lot of English. This is perhaps the most important thing about all this that nobody ever tells the newbies. In other words, for a very large proportion of people coming to Korea thinking they'll be teaching the English language, the reality is that they probably won't, really.