Don't be too harsh. People have their own busy lives and perhaps relationships are often made when they are convenient.
Also, too, sometimes the strength of the relationship is in a shared context — like co-workers. When a co-worker moves on, goes to another company, retires, there is less to chat about were you to meet up later, less shared context.
Your oldest friends though will often always have that shared context of having grown up with you in the same environment, same "era". You act as a foil to one another in the degree to which you turned out differently (also to the degree you stayed similar). Your eyes and theirs will gaze upon the same landscape from the same vantage point: "Look at those kids and their phones, we were out riding bikes when we were their age." (Ha ha, etc.)