Of course they are but they are meant for more sophisticated purposes, (i.e. install, update, reinstall, remove app or even purge program with user data, etc.) and all of that logic is usually covered with signature checking with other security-oriented algorithms. I agree, I find apt/dnf/pacman/apk/xbps are great, but...

In case of Ubuntu, in order to install some deb-based software you usually have to bring your own script (this is what I call "copy & paste installation script") to add third-party repository that contains given package.

well, the definition of a linux distribution is a repository + a package manager.

https://en.wikipedia.org/wiki/Linux_distribution

:)