Are they so different that it's justified to have so many different distributions? So far I guess that different package manager are the reason that divides the linux community. One may be on KDE and one on GNOME but they can use each other's packages but usually you are bound to one manager
Also, bit part of Portage (Gentoo "emerge") is being able to 'flag out' parts of the package out (or in) to the compilation.
Let's say you want to not have telemetry in your packages. So you set '-telemetry' globally, and each package that has known telemetry parts will not compile locally - so it can not be turned on (unless it's hidden really well).
Or you want to use pulseaudio? You can flag it globally, or for specific packages. That way you can influence software you install without knowing much about anything build-related - the work is done by the repository maintainers.
They won't be able to pry Gentoo from my cold dead hands. Arch, Nix/Guix can suck it, all my money goes to the Gentoo