Why wouldn't they. Nearly all are Lao and Burmese, and grew up where the roads are even more dangerous. ????
Nearly all grew up in neighboring countries. Cambodia, Lao and Burma, so would have grown up riding on much worse roads than Thailand. Most likely didn't wear a helmet. If you've never ridden on the roads in those countries, pure carelessness was probably more of a cause than drinking or speeding in many cases.