It’s because you’re not native. There are plenty of Indian/turkish/african/carribean - British people who working in Thailand as teachers. Because they are native speakers.
I was here 4 years straight in Ed visas, tourist visas and exempt runs and an 11 day over stay the same year. My remote job also confirmed that i was in fact working for them while on the previous Ed visa. My passport pages were completely full with visas from Thailand so I had to get a new book. They accepted me.