Its a daily routine for years. They're also not doing anything else, fighting is the career. No job, depending on age, no family responsibility either.
Also, plenty of pro fighters train 2, sometimes 4 times daily. Its not restricted to just Thais.
I think over here, alot of folks overestimate over training.
First time camp I did, I trained like a retard. Twice a day 6-7 days a week, 5x5, and of course doing the ridiculous starvation diet. I was eating around 800 calories a day (low carb as well). I didn't start to feel burned out, until near 4 months in. Mostly mental, due to the low caloric intake.
As I said, it was a retarded regimen, I've learned alot and would never do it again, and would steer anyone doing so away from it; Despite that, I was still able to push on under those circumstances. I had to reverse diet back, post-camp since my metab got shot to shit during that time.