Good thread and particularly interesting given the discussions we've had on other threads 
I think the main reason is that this is the way it's always been historically. Because there are 9 months between the act and the consequence as far as men are concerned, they have always had the option of buggering off and leaving the woman literally holding the baby. In the past 'sowing wild oats' was practically considered a rite of passage. When you've had that kind of mindset ingrained for centuries, nay millennia, it's not surprising that we've not got very far with overcoming it yet. Doesn't mean we shouldn't be fighting it though as it is absolutely terrible.
The thing is though, even today in a happy marriage, you'll still find that childcare is still predominantly the woman's responsibility. This is why I always get really really cross with people suggesting that 50/50 residency is the way to overcome this. It is only an appropriate response if the child was having 50/50 care before the split. If men really wanted responsibility for the children they would fight for more paternity rights, for more flexible working, they would volunteer to make sure that they are the ones to leave the office on time to pick up the kids, they would be the ones remembering that DC1 has a playdate on saturday and DC2 has a dentist appointment on wednesday that they the father would take them to. They would be the ones sacrificing a career to SAH or manage a realistic balance between work and children. A lot of men only seem to want shared residency when they realise that they may have to pay extra maintenance as a result.
The way forward that is best for the child is for them to continue in life as close to what it was before the split. So if Mum was the one doing most of the care, that's who should have primary residency. The way to make this FAIR and to get men to realise that abdicating their responsibilities is NOT ON, is to insist that they contribute financially - just the same as they were before the split but this time without getting their socks washed for free. Paying maintenance should be enforced with the full force of law.
FWIW, I think with the exception of conception, pregnancy, birth and lactation, men make just as good parents as women and I would like to see more of them doing that, but they have to fight for the right to do that WITHIN a relationship first before they seek to gain residency when a relationship breaks down.
Increasing paternity rights and making non-payment of maintenance punishable by prison would do so much to address sexism in this country, but it will need men and women to pressure the politicians into it as it is such a departure from the status quo.