I think TG and PG use a lot of busses/vans in part because it makes it easier to use the same aircraft for both international and domestic flights. I actually do not particularly mind it as long as everything runs smoothly. Indeed, sometimes a bus or a van gets arriving passengers to immigration faster than walking through the airport might.

I am sure another part of it is that Suvarnabhumi Airport would rather sell slots at the gates to foreign carriers for a lot of money...