It’s because the same people who wrote the code usually write the docs, and people who are really good at writing code usually aren’t good at writing docs. It’s two different skill sets that usually don’t coincide.

This is why companies ought to employ technical writers if they have enough documentation. Of course, few ever do, but it'd by the Right Thing™️ to do.

Here's a tip on good documentation: try to write the documentation first. Use it as your planning process, to spec out exactly what you're going to build. Show the code to people (on GitHub or on a mailing list or on lemmy or whatever), get feedback, change the documentation to clarify any misunderstandings and/or add any good ideas people suggest.

Only after the docs are in a good state, then start writing the code.

And any time you (or someone else) finds the documentation doesn't match the code you wrote... that should usually be treated as a bug in the code. Don't change the documentation, change the code to make them line up.