Or for that matter, why teach English as well?
For most Thai students the answer is obvious: it's a requirement. For many English teachers, especially foreign teachers, it's a job: a way to make money and keep their work visa current.
The harsh reality of teaching in Thailand
It's depressing just how many people come here to teach and leave with lots of resentment and bitterness. Or worse still, remain here polluting the place with those same feelings! Don't be one of them.