That's a good question; people tend to think that leaving your home country for a week or two is not a problem if you are physically fit, but when you are away from home, weather conditions can change, accidents can happen, and hospital bills are not cheap.
If you have insurance, it will cover you, so there is no need to spend money on hospitals.
Some people may become seriously ill and have to return home to evacuate, but if you have insurance, they can arrange for you to return home. It is difficult without insurance, so be sure to get it before you leave for Thailand.