In the case of the siblings I know really well, there are complicated reasons which stem from their childhoods and how they were raised.
As adults, they tend to keep their relationships with all people, romantic or otherwise at arms length. They are fiercely independent and although they may wish to be in relationships, at the same time they find them stifling and and a compromise to their sense of self.
From what I know, there is a desire to be in a relationship and it isn't a lack of offers, but more not knowing how to undo some of what was "programmed" while they were growing up to achieve that.
I would say that the siblings I know are naturally introvert, which I guess would have a bearing on this too.
Not sure I've done a good job of explaining this but am trying to describe it in a neutral way. Given the society we live in, it is really easy to make a judgement given our own frames of reference. If it adds anything, they are in good places in life, and are happy, sociable, loved etc.
Perhaps there is also an element of not settling for just anyone.