Because a Debian package doesn't magically work everywhere. It's not even guaranteed to worl on every Debian system, due to API/ABI breakages in Debian Stable v

Because a Debian package doesn't magically work everywhere. It's not even guaranteed to worl on every Debian system, due to API/ABI breakages in Debian Stable versions and then people also use Debian Testing and Sid. Hence if you want to declare support for Ubuntu 16.04, 18.04, 20.04, Debian 9, Debian 10, ... you either need to rebuild for each of those systems, or monitor ABI/API compatibility with proper testing, or link statically all of the dependencies you use. None of that is as easy as preparing a Debian package.