Most of the projects hosted on such sites are aimed at Linux, where a distribution has one package management system (such as
apt on Debian/Ubuntu,
pacman on Arch). Introducing a second, third, fourth package manager would cause confusion at least, and likely even conflicts. (For example: What happens when you try to install a program from MacPorts, but it has already been installed from Homebrew? Ugly things may happen.)
These distributions also have their own rules on packaging – configure options, locations of certain files, the package format itself; for example, Debian splits many programs into "main", "dev", "dbg" packages, while Arch does not. To build a single package for all distributions, one would have to conform to a logically impossible set of rules; hence each distribution maintains its own repositories.
Unofficial extra repositories do exist – many projects hosted on Launchpad have a PPA with packages built for Ubuntu; Arch Linux has an user-maintained AUR; finally, even Mac OS X has MacPorts and Homebrew :) However, all of them are similar in that they are specific to the operating system, but never to a website.