Because the n hours your wife is working is n hours not spent raising children.
But I am also working n hours less in the second scenario, so I can also spend n more hours raising children. Sure, there are things I cannot do (e.g. breastfeeding), but that's what maternity leaves are for.
Dalrock writes an interesting article related to Dr. Helen Smith's book the Marriage Strike. I really have to bump it up on my too rapidly growing reading list. (^_^)