One explanation would be, that men (on average) favor abstract subjects (STEM), and women (on average) favor social interaction (children).

(See e.g. https://en.wikipedia.org/wiki/Sex_differences_in_psychology#... - Childhood play, Intelligence, Personality traits, Empathy)

One point to consider that relates to the first year at least is breastfeeding.

Science very heavily favors breastfeeding above the substitute. While pumping breast milk is a choice, it can be difficult in practice and some science points to it still being different for both the mother and the child in terms of hormonal differences and of course the physican touch/emotional wellbeing.

There could be many reasons: Perhaps because mothers are more nurturing and have a more intense desire to take care of their newborn children, after all they shared the same body for 9 months. Another could be the hormonal changes postpartum, which might trigger emotional and physical changes, etc. Childbearing is no small feat.

At least in Asia, there's pressure for men to be providers (and they get socially disrespected if they can't). Not sure about the U.S. though.

How do society treat men who leave?

How do society treat men who stay?

How do society treat women who leave?

How do society treat men who stay?

Society in this case mean social status, divorce rates, size of social networks, number of children, and other measurable factors. Answer all four and we likely have a clear answer why men don't leave at the same rate as women after they have a child.

Because there is no social expectation for men to do so.