Anthropologists suggested we can only maintain about 150 social relationships at a time. That's quite a big number considering most of us choose to retreat ourselves to a few closer ones.
We further enclose ourselves from reaching out because of work commitments or because others do not appear open to socialize. Getting together with others soon began to feel outlandish. Ergo, we cease to find a new connection unless needed or obligated.
Old friendships fade. We don't like to deal with the tedium of re-establishing old relationships. As with many things in life, maintaining a tie requires ongoing nurturing, not a yearly new year greeting message out of a cultural duty.