Then there coordination overhead in the family - even if the two parents work 20 hours each, it's unlikely that it's exactly divided so one is always home. Child care costs a lot of money. Eating out costs money.
But the most important reason: People still want careers and a career on half the working time takes in extremely simple terms at least twice as long.
Sometimes you have to sacrifice to make do for your family by moving, whatever. It's always better to sacrifice money than time from your children. As long as you are making it and saving a little and have a bit for holidays, nothing else really matters. Getting rich should never be the goal.
Your tone seems to me like you once wanted to be rich, but have since given up and are now trying to convince others that they should just give up too.