Because there are rapid diminishing returns with every extra hour past the first few. My parents put a lot of work into me, but they were also full-time surgeons. My mum most definitely put a hell of a lot of work into me but she didn't spend all the time at home.

Worked out pretty well.

Of course you want to allow for people to do whatever they want, but part of that is enabling them to not have to be stay-at-home parents. And most people want to do something more than that, in practice, because creating things is a fundamental human need that most people have. If people want to be stay-at-home parents and they can do it, more power to them, but it's important to allow them to make that choice otherwise unconstrainedly and not through societal pressure to keep women at home because jobs are unsafe (there are whole categories of work one avoids at the margin in India if one is a woman, because you cannot guarantee safe transport from/to one's place of work).