You need to get out of Bangkok and take a look. You, rightly so, have based your assumptions on 2 or 3 areas of Thailand.
Expat teachers can be found all over Thailand.
If you worked (I note past tense) in an office with 90% expats in management, how many was that?
The "awful lot" of expats in Silom. You asked each and every one of them if they are working and what they do? That's incredible.