Always useful to make the effort to divide societies into 3: low, middle, and high.
Thailand has markedly fewer middle level people than countries in the top 10 of the HDI, so the likelihood of seeing them read a book, especially outside of a major conurbation, is small, hence the fact that this thread exists. If you're poor, you've got other, more pressing, things to attend to. I suppose rich people read too, when they're not consuming, driving too fast and just generally being appalling.
There's also the notion that the golden age of reading is behind us, brought about by fascinating screen-based tech' that compels us to look, click and swipe. I used to read loads, but since, say 2014, not so much. Looking back, that was when I got a phone good enough to read on. How ironic. Trouble was, it was also good at taking photos, and myriad other things, and soon I got sucked in like most others.
I used to read before sleeping, but then it changed to listening to podcasts. I'm trying hard to reverse this, as I tend to sleep better with the phone off.
Anyway, I used to see the odd person reading on the BTS. Mostly in Thai, but also in English, and probably in Chinese and Japanese too.