Debian based, arch based, rhel based are all somewhat different and have different package managers (with flatpak, appimage and snap that might be less important nowadays though)
Nobara comes with all the stuff for gaming, not everyone who uses Linux knows exactly what they need to install themselves
NixOS is fantastic and drastically different from all the others
NixOS, silverblue, vanilla are all immutable which makes a massive difference
Also not everyone wants to install their own DE, so if they want something like cinnamon, pantheon, KDE they need a distro that comes with it preinstalled