I think it's because people commonly forget what it's like to be a beginner and which things that are obvious to them are not obvious to a beginner. This is probably more true in Haskell where there is just so much that it's possible to learn that the most advanced Haskell programmers are very, very far ahead of the beginners.
I think most documentation sucks and I dislike trying to read it, because it's very hard to get a picture of what's going on, and what these procedures / data structures / etc are really for.
I am much happier when I can just look at a straightforward and clear example, and then just use the documentation to look up specifics of how things work after I already get the basic idea.
It's not because I "need worked examples of things to understand them", it's because that is the way I like to work, because I have had many instances of my life of trying to make sense out of documentation that seems to have been written from a mindset of "formal writing involves not actually telling the reader what things are really for, straightforwardly". I don't know why that disease is so common, but almost all documentation is like that.