You're assuming that bar owners in Thailand are rational (economically) and that small business owners anywhere are profit-maximisers. Neither is true. Small business owners generally are square pegs in round holes in the world of employment. They simply don't fit in. Therefore they seek "self"-employment
Bar owners in Thailand own bars because it's the simplest way of staying in Thailand and having something "productive" to do. Everyone is an "expert" on how to run a bar just by virtue of having stepped inside one once or twice - all you need to do to confirm that is to read threads such as this one - so they buy themselves a bar ... and it's all downhill from there