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 shocked.

For the vast majority of people entering Korea to work as teachers, the E-2 is what they will be applying for. If you are offered a job, you can either apply for and get the E-2 before you leave your home country, or come to Korea without a visa and do The Visa Run later. The most common destinations for visa runs are Fukuoka and Osaka, in Japan. Both Korean embassies are well-used to the constant, endless stream of E-2 applicants trooping through, and are quite efficient. The do tend to have odd opening hours, but if you time things right, you can leave Korea one day and be back the next, E-2 in hand.