As Pps have said, there could be a long list of reasons. From your posts it sounds as though she has confidence issues.
I've never been in a relationship (at age 27) - sometimes I feel I'd like to be, however mostly I'm happy in my own company. In my case it's down to awkwardness and lack of social skills. In my teens I was "odd", and now that I can "blend in" a little more I simply don't really get any opportunities as I don't go out much and work in a mostly single-sex setting - and I'm not the sort of person who would put myself out there or go looking for it. There's also always some excuse why it can't be now - I'm overweight, in debt, inexperienced. And of course the latter sticks out more the longer it goes on, and makes it more awkward.
But - those are my reasons, and there's every chance your friend is entirely different.
(Like neverhad I'm not sure if this is of any relevance/use, but thought I would add my 2p worth).
To answer your question simply, I think it's probably unusual, but perhaps not as much as you would think, and there could be many, many reasons. I definitely wouldn't jump to judgement.