It's also hard to get people to contribute to documentation. I had a conversation with someone recently that was frustrated about the Rails documentation: "Here's my blog post about how to do X. Apparently that's what Rails people do rather than write docs." When I asked why they didn't contribute to the docs rather than make Yet Another Blog Post that feeds into the exact problem they were talking about, it just hadn't actually occurred to them. Getting started contributing to a new project is difficult, posting something to your blog is easy.
Most people don't find writing docs to be fun, and Open Source is largely driven around doing what's fun.
It's hard to see the flaws in something you're close to. The people who are closest to a project forget how hard it is to come in from nothing; I had a conversation recently about Hackety Hack where someone was upset it didn't have the ability to make folders and multi-file projects, and when I tried to explain to them that the beginners (children and adults) that I teach actually struggle with things like file management, they were shocked. We all forget just how much we know, because it's easy to us!
Furthermore, there's an incentive for people to write blog posts over contribute to official docs, since then people will link to their blog, with all the benefit that comes along with.
I am not informed on the state of Angular's documentation, and I think Rails has pretty decent docs these days, but there's lots of reasons why docs are often poor in open source projects.