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.
Because it is not ok to live here full time on border bounces and extensions which will most definitely add up to a lot more than 10k baht with a once a year bounce and extension over the 5 years. It is ok to live here full time on the DTV, no questions on entry because it is multiple entry, that’s the point.