There are so many variables that come into to play when you have DC as regards whether it is possible for both parents to continue to work so hard and push for their career. It's never quite as simple as definitely a choice and there is nothing to stop you doing more with your life.
As other posters say, what if you have a job where career progression means actually working more hours, doing 70-80 a week? What if you have a job where you can't do any work from home or make up the hours in the evening? Then you could easily be faced with a complete choice of career v seeing the children. Not many parents would be happy with that.
You also have to factor in your partner's job. Mine is in the police. It's just not a job where he has flexibility - it's shift-based, it's unreliable, he can't drop and run for the kids as I can from my office job. This has an impact on what I decide to do with my career.
There is also travel and commuting, and how close to your job you can be. Unfortunately I have well over an hour commute each way to work, and it's not a job where regular work can be done from home or in the evening. If I went the next stage up to managerial level, I don't think it would work given DP's job, unless we had live-in childcare and we do not earn that amount of money.
In reality, actually, perhaps I have have stayed as ambitious because I have not dropped out of my city-based job - I am in a sector that is London-based, so no local jobs exist without me dropping out of my career path completely. As it is, I've stayed in the city job but am coasting at the same level, probably for the better part of the next decade.
Add in to all that the fact that as much as I want to work and keep the career I trained for, I also want to spend some time with my child, as does DP. I want a career but I can coast for a while and focus when my child/ren are older, because I'm not in a job at the moment where pushing all-put would be possible without rarely seeing my child.
Obviously everyone's situation is unique, but there are often dozens of factors that feed into your ability to continue pushing your career while your children are young, it's rarely very straightforward.