There are other reasons for friends to take on primary importance in a person's life, and they have to do with the nature of friendships and the nature of family, that is, what defines these relationships and how they form.
We are born into family, of course, and we do not get to choose those we are related to. Often times a person will have little in common with their parents or siblings (even at times, I have found, in the case of twins), but are nevertheless bound to them by blood relation, economics and geographic location, not to mention by law.
As we begin to mature socially, we are able to choose those we associate with, and human nature leads us to choose those who we share activities, experiences and opinions with. We choose those who reinforce our world view and reassure us that we are normal and accepted, whether we realize we are doing this or not.
For this reason, we tend to have more in common, are closer in age to, and can relate more to those we call friends than those we call family, and we begin to spend more and more time with friends than family as a person enters their teen years, and even more so after we move out of the house or go off to college. Then our primary group of friends replaces our primary group of family in terms of everyday socialization.