We can throw personal preferences and anecdotes around for days, but as a simple matter of fact Ultra High Net Worth Individuals move to
1) Rich, English speaking countries like Australia, the US, the UK, New Zealand (and Switzerland)
2) Warm European countries (Isreal, Portugal, Spain, Greece, Malta) or the Caribbean
3) Developed Asian city-states, namely Singapore and Hong Kong
Source: AfrAsia Bank - Global Wealth Migration Review 2018 & 2019
Asia overall - China being the frontrunner - has a large net outflow of these super rich people, and Thailand is likely no exception though there is no substantiated data that I'm aware of.
Now a lot of reasons have been mentioned but Thailand is simply a good place for people with modest means, but not so much for really rich people. As a single digit millionaire it certainly is an option, if you have more money than that it's not. "Cheap and fun" nightlife just doesn't feature highly on their priority list, nor do tourist beaches. I honestly struggle to think of any advantages or benefits living in Thailand would offer an UHNWI. Occasionally visit? Sure, a couple of exclusive hotels and private islands offer a pleasant enough stay.