Because they know they have the power. The woman is physically weakened by pregnancy and birth, and is responsible for a new human being who requires care 24/7/365, which will impact her ability to protect herself, work, everything. Pregnancy and childbirth make women vulnerable.
I think that the way society treats mothers is part of it also. I remember how differently I was treated by other people (doctors, nurses, randomers, everyone) after I got pregnant, and then it changed again (i.e. became worse) after I had my baby. Completely random people felt entitled to make comments on my body, tell me what to do, and all sorts. And after I had the baby it got so much worse. I remember laying in the hospital bed, paralyzed from the waist down (C-section) and the nurse telling me to change the baby's diaper (which was impossible, as I couldn't feel my legs and therefore could not get out of bed to get the baby) while my husband was standing right next to her. As far as the hospital staff were concerned, the baby was completely my problem and my husband was free to wander off and live his life. It was as if I had been magically transported back to the 1950s. New mothers are the lowest of the low. Everything that goes wrong is their fault, people talk down to them, criticise them, judge them. This sends a message to fathers that it's ok to treat the mother poorly and that the mother is probably the cause if anything goes wrong.
I think religion also plays a part. I am pretty religious but I do think that many men, when their partner has a baby, suddenly become religious, but ONLY in the part of religion where women are supposed to submit to men. They don't really remember anything else, just that part.
Just my opinion.