The question is "why don't lonely people interact with each other".
The answer is that being in need of emotional comfort does not imply the ability to provide it, so after some time they are bad fit even for each other. The same goes with "why don't unattractive people date each other".
I think as people get older, after 30, they become set and comfortable in their world of loneliness. The focus narrows so that they only look for the perfect, the one "solution" to their loneliness; "the one." The focus becomes singular when the solution to loneliness is multifaceted; get out, do activities, meet many people, etc. Ergo they remain lonely.
A face value may sound like an absurd joke but it’s 100% the truth. The unfortunate reason many people do not see it this way is that it is a difficult skill to practice if you’ve built a lifetime/identity ignoring it.
The answer is that being in need of emotional comfort does not imply the ability to provide it, so after some time they are bad fit even for each other. The same goes with "why don't unattractive people date each other".