The point is to get patches accepted without forking. Linux and glibc aren't exactly small. Do you honestly see a single man maintaining millions of lines of forked code as well as handling packaging and stuff?
It's not as bad as it sounds, basically great many of the http://git.kernel.org/ repos are actual forks of the kernel, regularly pulling changes from vanilla, resolving the occasional conflicts and maintaining/developing their own patches.
Actually it is as bad as it sounds. Although git makes it easier to merge with mainline, in the end it still requires an active maintainer who manually merges and tests stuff once in a while. Maintaining a fork requires one's constant attention and diverts one from doing other - maybe more useful - things.
