For most of humanity a family group stayed together for the majority of each day. Children played with siblings, cousins and were usually within earshot of one of their parents or relatives. Certainly they would have found their way back when hungry. As they got older, they worked right beside their parents on the various chores and tasks.
In modern western society, the spouses are separated for the majority of their waking hours. (Even if one spouse chose to stay at home, the issue would still remain as the other spouse would have to go to work). The children spend either a significant or a major portion of their time in day care or school, also separated from their siblings. Then throw in the after school activities, projects and homework needed to create a modern "well rounded" child. (Don't even get me started on how un-affordable it is to have kids)
They do get to have a rushed breakfast together and hopefully a good dinner as well, but that is about it. A significant portion of the weekend is spent doing chores. Add in the American working culture of taking very few vacation days.
The decline of the family is because our culture has created an environment where we are working .. working constantly.. (either chasing a higher standard of living or running as fast as we can just to maintain our current standard of living (housing, education, healthcare), working so you can afford to have kids) and we work in an environment where we are disconnected from our family and friends. Often, if you are lucky, you make friends at work.. but almost always these aren't like the friends you made at high school, they aren't as meaningful and strong. Then People change jobs and you "stay in touch"... If you are lucky, you find the person you love at work, but work places romances are tricky as well.. sometimes (usually?) frowned upon.
We just have much less TIME to build those sorts of deep bonds. Visit a village or a small town in the developing world and observe how much more TIME people seem to have. How much more time they spend with each other.