In full disclosure, I haven't rtft. However, if all SAHP went out to high "achieving" roles/careers, would you then be saying "why aren't all those doing childcare trying to achieve more?" Or for that matter cleaners, janitors, waitresses, waiters, the lower end healthcare professionals, TA's, street cleaners, all the lower paid but completely necessary professions that aren't seen as "important". Not everyone can be high achievers, society doesn't work like that!
Of the SAHP I know half are men with very high achieving partners. Your argument also doesn't take into account any emotional/mental health issues that might occur from having given birth.
My talents don't lie in the medical field, they lie elsewhere. However in my field it's all or nothing to get ahead. All is never seeing my family, I've never wanted that and chose to remain good, employed but on MY terms that allow me to spend more time with my family. This also means I won't be a high achiever, I won't earn a great deal but I and the entire of my family are happy.
Also you don't take into account elderly or ill parents....