The thing is that not everyone considers the medical insurance costs.
Once you hit 60 full medical goes up to $3,500 per person. So $7,000 for a retired couple. If you are planning on staying for 10 years, thay is $70,000 (2,400,000 Baht) in insurance premiums. That is why many self insure.
Plus if you do get sick your insurance preimums go up after you claim.