Depression probably evolved because it aided survival in cold climates. In hot climates, people and animals can move around easily; they need to keep finding food. In cold climates, people get snowed in and need to conserve energy; there isn’t any food on a snowfield anyway, so there’s no point in going out and looking for it.
Social depression reminds me of a hibernating animal. People who become depressed and who stop social interactions have often run out of the necessary physical and mental energy to go out and engage with people socially. Shutting down to social interaction doesn’t necessarily require shutting yourself in your apartment and watching anime. Shutting down to social interaction can also include when you walk down the street and make a small amount of eye contact but you are not actually open to emotional interaction with anyone on the street.
If you’re being actively harassed by peers, this kind of emotional shutdown doesn’t give you a reason to avoid going to school, but minimizes the emotional energy lost to peer interactions.
Of course, once you get into this shutdown state, you’re not likely to get more emotional energy that might allow you to re-engage with the world. So depression can turn into a trap. When you give up on emotional relationships with the human race, you give up on a part of yourself.
Even if you manage to re-engage with the world and solve problems and achieve goals, a long period of disengagement from social reality can hurt you.
And here’s the problem: socially disengaged depression is not just a problem for a few teenagers who wear black lipstick. It’s a problem for whole societies. The whole planet is set up so that Bill Gates can sit on a throne of banknotes and preach about how his vaccine projects make him morally superior. He can get away with that for various reasons; one reason is that the common folk are too socially disengaged to stop him.