Because docs take time to write, and good docs require a passionate dev who cares to write them
Sure, if documentation is treated as an afterthought it tends to reflect that attitude.
Bingo.
Do note that people here on HN live in a bubble where, for example, writing tests (any tests, not even good tests) is a given. But out there in the world there's plenty of software coding, a lot of it in major companies, where developers think testing is some cute but useless thing they teach you in college and which can be safely skipped, and managers are completely oblivious about this. Same with writing useful documentation.
For documentation to remain relevant there needs to be some kind of process actually checking every part of it against reality.
I mean, with actual code you get some help from the tools. No silver bullet, but at least you have type checks, compiler errors, something. But with docs there's no way to automate checks to see if the doc is still relevant and accurate. And what terrible tools are there tend to lead to "boilerplate docs", like those javadocs mentioned in a comment elsewhere.
I've been wanting some kind of system to add references to tests to documentation, in the spirit of citation. Maybe I will build it at some point.