Yes, but why take up space in the actual source code and repo? It increases space usage both in terms of disk space (which means more download time) for the lib

Yes, but why take up space in the actual source code and repo? It increases space usage both in terms of disk space (which means more download time) for the library and requires more scrolling when reading the code. Also, if the library is a sub-dependency which the developer doesn't interact with directly, why should they download the documentation for it? They will never read those comments in the code anyway.