When 50 percent of the population is involved in agriculture and produce 10 percent of the GNP. That is inefficiency. When a Thai person's education is required to 6th grade. That is another measure of being underdeveloped. When pollution from manufacturing threaten the health of their populous, this is a sign of a nation under-development. These are the reasons why I see Thailand on the rise towards being developed.
And one reason that it may take a lot of time for all to share the wealth is the corruption that is rampant in Thailand. I believe Her Royal Highness, the Princess of Thailand and or the Queen have made it a special project to better the education system in Thailand. I applaud their work.
Oh yes, it does not help that a few large corporations or conglomerates hold most of the wealth.
Take a look at the Philippines, as much money as the USA sent to the Philippines, only a few benefitted from it because of the corruption in the system.