Storybook as an official documentation tool is a fairly new idea. They added Storybook Docs pretty recently, which is basically stories that can be written in MDX. It also enables you to document a component's props among other things. IMO, Storybook Docs is on the right path and will become a great documentation tool.

But just using straight up Storybook as documentation, I agree that isn't very good. Whenever I'm just given a random Storybook, my success with that library has been pretty low.