My salary pays the mortgage, food, utilities, .... I optimize for keeping things payed for and running, my wife optimizes for family health and happiness. I'm not saying that is universal, but that is how it works for my family, and a good number of other families I know.

Because it's not socially acceptable to take a hiatus _instead_ of the mom and not economically viable for both parents to take a few months (or ideally a year or more) to take care of the newborn. Because men can't breastfeed. Because mothers are more attached to their children, instinctively and psychologically. There are dozens of reasons "why".

That's a great question, there are so many overlapping factors, such as not even having the right, or not being able to afford both parents on lesser-paid leave, to basic stuff, like they don't do the breastfeeding, or social stuff like there being no "mum's club" for men. That's before you even look at culture but I suspect that when people are faced with financial and logistical limitations then culture and preference are actually not the first place to look for answers.

I'm on paternity leave in California right now and if it wasn't for my wife's employer offering benefits far above those the government offers, we wouldn't be able to afford it.

Two things I've observed so far are that with my wife breastfeeding, she keeps her days full, but I'm bored out of my mind. It's not like the baby does anything other than eat and sleep. The other is that new mothers are typically accompanied by their own mothers - so my wife has her family around, which is great for her - but it's not the same as having my own family around.

For our family, I didn't take any significant time off while my wife did because I genuinely love my work and it's very highly compensated, both of those factors significantly in excess of my wife's job at the time.

She loves the outcome; I love the outcome; the kids love the outcome. It was about as close to a no-brainer decision as I've seen on an important topic.

Because women are paid less, so couples decide the best paid member will keep working.

Good luck getting paternal leave in the US.

I took one (by being laid off 2 days before my child was born). I don't recommend that method, although I'm not sure how else I would've gotten 6 months paternity leave to help out at home.

The ladies at work here seem to be out for 1-3 months after giving birth, in most cases.

In addition to other reasons, two kinds of leave are conflated in the US system. There's the sort of leave that occurs when the kid is born. Women typically can spend days (sometimes more) in the hospital convalescing after a birth. Leave policies don't really distinguish between "leave to have a baby" and "leave to look after a baby," it's all lumped under "maternity leave."

Is it even a benefit to work instead of taking these hiatuses?

Parenthood can be a vacation or hard work depending on whether you are a good or bad parent. Just being a parent does not mean that it is difficult work. And if it is hard work, it can be easily argued that being a parent is more meaningful work than 99% of corporate jobs.

Because it's not often expected or encouraged for fathers to do so

Because paternity leave is far less common than maternity leave. Because women expect to be supported during and after pregnancy.