Simple answer, because it's a single OS with a single focus and centralized development. Compare the Arch WIKI to the OpenBSD docs and marvel at the difference

Simple answer, because it's a single OS with a single focus and centralized development. Compare the Arch WIKI to the OpenBSD docs and marvel at the difference in scale. There are downsides to it too, Linux has a lot faster development and broader compatibility but this also results in more complex amd harder to maintain docs.