On another note, why can't somebody in a higher position be friends with a servant or someone who works for them; it seems like prejudice and distasteful to say the invisible line should never be crossed and never treat a servant as nothing else but that.
It is a very old custom and will not be anachronistic as long as there are people: the very idea of friendship means some sort of equality which is difficult to achieve as long as one orders the other around. Normally a situation like that with two different scenarios interchanging at different times (master-servant, friend-friend) leads to a different view of where they stand at different times. And that must not but in most cases leads to trouble eventually. Which prudent people best avoid.