Is it that unappealing for US citizens to get a STEM PhD? Especially in the US the private sector ought to offer enough jobs at the PhD level and pay to make the PhD worthwile.
Depends on the particular stem field but in general, if the goal is a well paying job, STEM PhDs are one of the harder ways to get there.
Hard ? Because of getting the PhD in the first place or refocusing your career afterwards ?
Well, again, it depends on the particular field, but both reasons can apply. My study of black holes was hard and didn't exactly make for a ton of usable skills other than knowing how to learn.