In my opinion, the origin of these biases is in the early, formative years: When children see mom stay at home and dad go to work, they learn that that's the way things are - without any conscious negative judgement and regardless of gender. It's the society they grew up in, so seeing something deviate from that can create a bias. It's naturally self-propagating as well, so it will take conscious effort (from everyone) over generations to overcome.
I highly recommend taking the Harvard Implicit Gender - Career Bias test (https://implicit.harvard.edu/implicit/takeatest.html). I found it extremely enlightening because I could actually FEEL my brain pulling me to answer incorrectly. It's timed, so there's mental pressure to make quick decisions - where the brain takes advantage of pattern matching that's been trained over a lifetime.