They're almost 100% interesting, but they don't like talking about themselves or hide secrets, etc., which are interesting traits, if not difficult to crack open.
I feel like it would be better to phrase that as you found them uninteresting.
Or did you mean they are un-interesting in a way most of society would say someone is uninteresting. Like they aren't interesting because they don't follow sports or TV shows? Or they only talk about work or kids? Those people can still be interesting, just to different types of people.
They may not be interested in being interesting for other people?
It's very tough to describe people who aren't interesting in order to tell you why they're not interesting, now I've set about doing it. It's like describing a colour.
Well, in the real world where normal people live, no, but philosophically, fine yeah. But then if you want to be philosophical I'd go further and call that equivocation between "interesting: piquing my curiosity in a way I like" and "interesting: some remark can be made", and I don't find the latter interesting to think about anyway.