Are you genuinely having difficulty conceiving of a friendless person? Because it would be fairly easy to perceive a certain negative tone in some of the questions you pose, or at least an inability to empathize.
For a long time after I moved to Oregon, I had no friends (unless you count my SO, to whom I was not married at the time). It’s not just one thing - it’s very complicated. (1) I’m quite introverted and in fact do have a mental illness (one that makes trust difficult for me), as well as social awkwardness. (2) My SO is extremely jealous, untrusting and controlling. (3) It’s much more difficult to meet new people, especially those with whom you are compatible, when you work for very small businesses, as I have. (4) I am a childless woman who is not at all interested in the world of children and child-rearing, which means parents and I have a large area of their lives that can be a hindrance to friendship. (5) I have physical infirmities which leave me exhausted by the end of the workday, thus negating the possibility of me getting involved in many after-work activities like sports or volunteering. Some days, I barely have the energy to get ready for bed. (6) My tastes in just about everything are a bit out of the mainstream - I don’t like most of the same music, TV shows, books, etc., that are most popular with the larger portion of society, so that’s one less area of common ground on which to establish a relationship. It’s not something I’m willing to compromise on, because I have to be true to myself to be happy. (7) My definition of friendship, while not strict, does not include everyone I’m acquainted with on decent terms. I have to actually chose to be in their company and enjoy it, not just happen to be thrown together by circumstances and such. For instance: there are people I know from church, chat with, even go to home groups/Bible studies with - but I’d never have them to my house for dinner or go shopping with them or whatever. There’s just something about them that doesn’t “click” for me. (8) I’ve lost touch with everyone I grew up with. I’ve tried to keep in touch, but some of them just don’t respond any more, so I finally gave up.
Which leads me to my point: friendship is based on chemistry in the same way that love affairs are. You can make an effort to be friendly to someone, but you can’t make yourself feel those warm feelings if the chemistry just isn’t there.
I do happen to have a few friends now. It just took a long time, due to the obstacles above. But I understand what it’s like to not have any, and would never presume that a friendless person “just wasn’t trying” or was otherwise at fault. I hope I’m wrong, and that you aren’t making those assumptions, either.