If you are not partnered and you want to be, then logically you have not found a person who appeals to you or that you feel comfortable with, therefore "the right one hasn't come along".
1) unknown origin
2) only if they want one. If this is said by someone else, it's usually a comment made sort of poking at the unpartnered one for still being single...often by a parent who may be old-fashioned enough to think that marriage is the only way.
3) no of course not
4) It's neither. If it's said by the single person, it's a comment on their own single condition and can often be an excuse if they feel they need one in an awkward social situation such as meeting relatives of married friends who might be judgemental
If it's said by someone else, it's again more a nasty comment on a person's single status than anything else, but it can also be a sort of standard comment by older people as just sort of a social thing to say when introducing people or talking about people. They just don't realize that it is not really polite.
In actuality, the "right person" is probably what most people are looking for - someone to connect with who fits in or where they fit in with a life-style, a cultural style, a personal style so that sharing each others lives is made more possible than if they connect with someone where there will be conflict. This doesn't mean that they have to be the same, just that they can connect.
For example it would probably be impractical for a person of fundamentalist muslim faith to partner a fundamentalist Christian - they would both believe the other was wrong in their beliefs.
A person whose life was totally involved with music might find it difficult living with someone who hated music.
Hope that helps