There's a difference between friends mates and acquaintances and there's different kinds of friendship too. Deep question. Not sure I've got the mental capacity to explore more deeply atm. 

My oldest friend I've known for 28 years now. I've got mates I grew up with who I've known considerably longer, but they're not part of my day to day life. But there's maybe three people outside of family who, if something were to happen to them it'd  have a significant impact on my day to day life.  

 No need to be deep to be you.
I fully understand what you mean i think most of us whould say we can count our close friends on 1 hand  

 The quality of the friendship does not depend on its length. 

So many people we’ve known, and we’ve called friends for a long time. 

but in the reality we just shared common circumstances that made us friends, when those circumstances no longer exist, those friendships fade away.