I've been a teacher in Asia for half my life (but not teaching in Thai schools.) I have seen companies and schools hire people to teach English who;
... were not Native English Speakers.
... had serious speech impediments (stuttering, strong accents, etc.)
... had zero experience or training to be an educator.
... were taking the job just to stay in the country with a valid visa.
... had the 'idea' that they could communicate well, but in fact, didn't.
... nice 'kids,' but no idea how to reach people of a different culture.
Even as a university professor, I watched other lecturers show daily movies in French (to an English class,) another who just gave the students a 5-page writing assignment as their entire semester's work, and another who only wrote on the whiteboard, but never spoke in class... and this was university level!
That said, I've also met some of the finest ESL teachers! People who take their craft (yes, teaching is a craft) seriously, continually working to refine their skills. Educators, not job-fillers.
People who are in it for the outcome, not the income. Certainly not for the income. Not in Academia.
The problem is universal, not just Thailand, although Thailand does take it to another level.