Close passing is, I think, is sometimes genuine ignorance from people who drive everywhere and have no comprehension of what it feels like. If you've ever walked down a narrow road with no pavement, you'll be familiar with the same behaviour*.
Other times, it's just a case of simply not caring. They want to get ahead and they if they can squeeze past they will. It's viewed as a right. And as no one will tell them otherwise, well, why not.
In combination, well, that's passing behaviour. It's never that they can't see you or need to be told. It's like the people who ride around after dark lit up like Vegas. Trust me, it's not that people couldn't see you, they can see a bike with a single decent light well enough, it's that for whatever reason they've constructed they simply don't care or aren't looking.
*there are three types of country lane pass in my experience: the wide and slow, the wide but without slowing, and the don't care. The latter seem to be winning out these days.