They test that massive amount of packages to make sure the dependecies and compatibility are kept. They do that between DEBIAN packages. The maintainers of the bash debian package can't just shove a new release in the repository. It's tested in DEBIAN systems first.
Who would test this new bash package in every fskcing "addon distro" that installs on top of "core debian" before releasing it to the debian repository? Or would the maintainers of every fscking distro have to scramble to update their packages after debian released this, and users have updated, breaking compatibility with the "addon" packages?
Or the opposite, the "addon" distro package developers want to use a new feature from a library, but can't, because debian hasn't updated their packaged version yet.