These things may differ from place to place, but in places I know most teachers get advanced degrees because that is one of the very few ways one can increase t

These things may differ from place to place, but in places I know most teachers get advanced degrees because that is one of the very few ways one can increase their salary. Some get in order to get promotion to higher administrative-managerial, non-teaching positions. Prospective teachers get one for increasing their prospects of getting hired. Many of them may actually not even get advanced degrees in the very subject they teach, especially if it is a harder one. They often choose fields like special education for which there exists an established industry of producing degrees in some places (moreover devaluing the degrees of the ones who actually study special education), or anything remotely relevant, but actually not. From what I briefly saw in some of the articles, the researchers don't check about specific degree field but "educational attainment" in general. In general, quite a few people probably do not get degrees in order to get in depth knowledge on the subject they teach. This, along with other factors, makes imo the conclusions more about the current "credential" system than whether actually getting more in depth knowledge affects teaching. Of course, the educational system has a lot more problems than teachers not having master degrees in their fields, but going from anticredentialism to asserting that getting formal advanced (or even any) education in a field is useless is not imo warranted.