"It doesn’t even seem to occur to people that one might pursue a doctoral degree because one is interested in the subject and wants to do research. "

Frankly, no. Because as beautiful as that concept is, it's 6 years in your younger decades, where the magic of compound interest makes every dollar you save or spend count a lot more than any money you make later in life.

Add tuition and loans, also with compound interest, and well, I have no resentment against people from wealthy enough backgrounds that they can just do this. If that's how want to use your privileged background, nothing but respect from me.

But 99% of us have to consider the money.