Is there any value in having different paternity vs. maternity leave or primary vs. secondary caregiver? Wouldn't something like X weeks paid for anyone parent

Is there any value in having different paternity vs. maternity leave or primary vs. secondary caregiver? Wouldn't something like X weeks paid for anyone parenting/birthing/adopting. (i.e. a paid family leave) be a simpler design? You could expand this over time to include caregiving for elderly people etc. And it could support various family types (i.e single parents, adoptions, same-sex couples, surrogates etc.) Genuinely asking because it seems weird not to have just a simple inclusive policy.