It’s not considered the same because it’s not the same. In traditional societies, kinship networks allow this web of interdependency that just makes life easier. It’s not just the depth of the relationships, but the breadth. Your friends are probably the same age as you, work in similar jobs, etc. That’s not true of your kin, who will be in a range of aged and different life stages that can support you, or whom you can support.
Two of my mom’s sisters lived with us between college and when they got married. How many people do you know that are say 5 years older who would let you move in with them for years? How many would watch your kids every day for months? How many of your friends are going to attend your parents’ funeral?
I would ask those same questions about cousins. The funeral one seems irrelevant/circumstantial, but the others imply a person who is willing and able to make personal sacrifices for another person. I'm fortunate to have many of friends who meet those criteria, and maybe one cousin that does.
I'd also say that though friendships tend to be very homogenous by age when we're young, that can change considerably as we get older. But because longevity is one of the key factors in any relationship, most of my closest friends are +/- 3 years of me. And in support of your point, it's often when we're young and mostly have young friends that we need support, and those friends are not likely to be in a great position to help.
EDIT: I actually did get help from friends' parents as a kid! As well as an adult "family friend". Those types of connections shouldn't be overlooked.
And yet most of humanity's scientific and technological progress happened in a part of the world (Western Europe from about 1500 to 1900) with relatively weak kinship networks by world-historical standards.
While they could have been relatively weak, I would not presume they were weak or played a small role in progressing civilization. Being a cousin was recognized as being important for lots of things at the leadership levels and I assume that filtered down to society:
> Traditionally, many factors were important in arranging royal marriages. One such factor was the amount of territory that the other royal family governed or controlled.[4] Another, related factor was the stability of the control exerted over that territory: when there was territorial instability in a royal family, other royalty would be less inclined to marry into that family.[4] Another factor was political alliance: marriage was an important way to bind together royal families and their countries during peace and war and could justify many important political decisions.[4][116]
I’m just pointing out that friendship networks are a treated differently than kinship networks because they are. Hard to deny the economic effectiveness of the western model, however.
Two of my mom’s sisters lived with us between college and when they got married. How many people do you know that are say 5 years older who would let you move in with them for years? How many would watch your kids every day for months? How many of your friends are going to attend your parents’ funeral?