Because you don't want to use your system's package manager apt, pacman, or whatever.
Or use better software. Or use better languages. I don't have 11 versions of ffmpeg installed despite it being required (or optional) for 11 other packages.
debian repositories have 23 168 packages, all languages included.
Or are you suggesting each python package author should release one version rpm, one deb, one nix, one msi, etc? And then that the deployment script branches for every single OS isolation isolation there is?