Because most problems are not self-correcting -- they require real and conscious effort, and even that can be futile if it goes against larger forces (e.g. economic downturn / necessity).
Poverty, sexism, discrimination, bullying, corrupted politicians, bureaucracy, the examples of not self-correcting issues are countless.
Sexism, discrimination, bullying -- there are no aligned incentives on opposite sides of these, by definition, so no expectation whatsoever that they'd be self-correcting.
If 20 lonely people are sitting in a room, no one's going to stand up and say, "Hey, I bet a lot of us are lonely. Why don't we hang out together?" Even though they're right that everyone is lonely, what are the chances they'll all get along? Or maybe one person goes around to the 19 others and individually asks each one to be friends. The success rate on that is going to be rather low, and it'd take a lot of time.
Your rhetorical question is misguided because it presumes that loneliness implies an already existing effort ("quest") to be less lonely. I think it's safe to say that most lonely people have no idea how to go about being less lonely (and if they do have an idea, it's daunting to them).
There's a useful distinction to be made between lonely and just being alone. Alone is the state of not having anyone else close to you in that moment, lonely is a negative emotion that comes from being alone.
It's quite possible to enjoy being alone, I know I do, my mind is much freer to wander, and I sure I'm not the only person that feels this way. As Byron put it...
"There is a pleasure in the pathless woods,
There is a rapture on the lonely shore,
There is society, where none intrudes,
By the deep sea, and music in its roar:
I love not man the less, but Nature more"
There's also a style of alone that can be very enjoyable, which isn't so widely commented on, and that's to be part of the hustle and bustle of a big city, with lots of human activity around you, but where you're only involved as a passive observer. Buses are good for getting in this state, as is exploring a city by foot (even better in a country where you don't speak the language), but really it's a mindset that's available anytime.
That said, if someone wants to overcome loneliness, it's possible for them to make their own opportunities to interact. Finding people with shared interests is easier than ever, now that we have the Internet.
After 30 you are dead to your genes. The machinery that drives you before then expires, and you have to find another way.
It's pretty hard.
Those take much more than the entire recorded human history to change...