Almost every package manager on Linux distros allow you to install packages outside of the main repositories and still manage dependencies using the repos.

I agree there are a lot of distros to package for, but I'd say the bonus of not having to maintain and update all the dependencies outweighs that fact.

For example, there are a lot of tools that can generate Debian packages for you if you don't want to spend any time learning the manifests. I assume these exist for most distros.