I could understand if someone came on holiday and got McDonalds or KFC, but if you live here I don't know why one wouldn't eat fast food. I didn't come to Thailand for the food. I can't really say I've had any thai food that was amazing, it tastes good but it isn't something I'd eat everyday. The only Thai food I have found that tastes great is mookata. I am asian though so a lot of the thai food is just a repetitive version of another asian dish so I've been eating it my whole life (even mookata).
It's a convenience as well, you have McDonalds/Subway/KFC everywhere, it's actually hard to find good thai food unless you live in central bangkok, it's usually hit or miss where I live now and the quality/cleanliness isn't great (yes fast food is going to be cleaner just ask trump). Most malls have more international food than thai food and I live by a mall so choices tend to be non-thai food.
Probably due to overeating more so than the food itself.
Thai food isn't very healthy to be honest, it's one of the reasons I don't eat it daily. Lots of sausages/ processed meats as well.
Usually when I do it's mookata or som tom now, the other dishes have too much oil.
Curious what do you usually eat in Thailand?