Some software is brand new and not yet packaged as software packages. And for already existing software Linux distributions will have to make choices as not everything can be included and maintained. Now if a developer creates new software, and things are not packaged yet (With Debian for Debian stable this can take a really long time) it can be comfortable for the developer to provide just instructions about how to compile the software so users can run the software, while the maintainer does not have to bother about packaging.