Me, I did notice them and even care but not much anymore.

I've known a few of these people, after being lonely for too long they usually develop some very sad line of defense that is 'There is no need for friends or social life".

Rare times in my life I ended up inviting lonely persons spending time with me and a group of friends (pity), I thought I was doing them favors by showing them how good a social event can be, but I did not help them, I did the very opposite, because that afternoon they are very happy, and they can hardly hide their constant over-smile, but the next day their whole lives are reshaped into feeling more horrible than before for not having built friendships through so many years, it's really depressing. especially that that fake line of defense that was created by their brain is debunked and is no longer there to protect against that sadness and loneliness.

I really blame their siblings and their families and their classmates and their neighbors and just everyone who knew them at early stages of loneliness and did not play an effective positive role.