All of these countries do support multiple, sizable airlines in addition to their flag carrier. However, in most cases the role is filled by a very successful LCC: Air Asia in Malaysia, Cebu Pacific in the Philippines, VietJet in Vietnam to name a few. These airlines have done a great deal to stimulate air traffic and provide alternatives to the flag carrier. Thailand does have Bangkok Airways, which is not an LCC but claims to be a "boutique airline" whatever that means. In any case PG doesn't really do much of what a genuine alternative to THAI would do.

It's also worth noting the number of European countries that can only manage to sustain one major airline: Germany (Condor is a leisure carrier, not a real alternative to LH), France (Transavia is owned by Air France's parent company so isn't a genuine competitor), the Netherlands (ditto about Transavia), Italy (which can't even seem to sustain an independent flag carrier anyway).

Or consider Mexico (Volaris is an LCC, not a full-service competitor to AeroMexico) or Canada (WestJet tried for years to become a genuine alternative to Air Canada before apparently giving up and retreating to Calgary.)