Two thoughts on why people go for doctorates even though they are unlikely to get a job that requires it.

One is that there is a real conflict of interest for their professors. They are supposed to be advising their students in their best interests, but they need to keep a big surplus of graduate students to keep their graduate programs going, and often also as research assistants.

The other is that American elementary and secondary education is designed to funnel people into higher education, and so that is what students learn how do do, rather than getting a job in the real world. Contrast this with Germany, where there is a very solid track for learning useful skilled manual trades (and that is a key reason German manufacturing continues to be very healthy in the face of Asian competition)