The problem with documentation is that the infrastructure/tooling for it is terrible compared to the infrastructure for code. Documentation is often in external systems (Confluence, etc) which lack version control and are hard to keep in sync with the code, and those systems are often extremely slow (especially considering all it does is handle text) and the user experience is terrible (no Markdown support, mandatory "wysiwyg" input, etc).
In all the places I've been that's now how things were and there wasn't any interest in changing it, so instead they kept going with a shitty and outdated Confluence/etc.
