Problem is, "better" is not one-dimensional.

Some foreigners date Thai that are almost certainly better looking and initially treat them more nicely than those they could date at home. However, back home, partners speak the same language and share the same cultural assumptions, so there's a higher chance they'll communicate and "read" each other's motivations effectively (though it by no means always happens).

With that pretty and nice Thai woman, communication is low, their motivations completely opaque. Even if you push past the language barrier (which many couples don't manage), cultural problems remain. Thai culture is big on assumptions and bad on open communication, so even perfect language proficiency and some cultural awareness often doesn't help.

If the couple live in Thailand, this lack of communication and understanding is more of a problem for the foreigner than the Thai. The Thai partner retains her family, friends and support network. The foreigner often uproots and moves to Thailand, with his partner being virtually the only person he can rely on.