Earlier quoted context omitted.
We already suffer from declining birthrates in the west. Your solution would make not having a child the best choice for "go-getters."
It already is for female go-getters.
No matter how you slice it, government mandated leave that doesn't apply to every single person equally will lead to discrimination. If you mandate maternity leave then companies will prefer young men to young women, and if you mandate paternity leave then companies will prefer the old to the young.
Parental leave is detrimental for every person that has it, but doesn't want it. If you're of child bearing age you're a huge risk to your employer if your resume doesn't come with a sterility test stapled to it.