Remember that the ranking is achieved primarily through English language journals. Much of the Thai rankings are deserved, because they are strongest in their contribution to English language medical journals. Those articles are from English proficient Thais. The engineering articles might do likewise. Other than that, for things such as social sciences, humanities, economics, and business, they are going to be relying on imported Western faculty primarily from English speaking countries. Few Thais publish in fields such as that. Look at the Thailand specifics, here: https://www.scimagojr.com/countrysearch.php?country=TH