It's how git was created to operate. The linux kernel, probably the largest software project in history, only operates through emailed patch files too.
Linux uses pull requests, too, it's just that it uses them the way they were meant to be used—among a limited cohort of collaborators frequently pulling from one another—instead of GitHub-style pull requests that have little to do with Git-the-tool and that everyone gets shunted through, even when a patch (or three) would be more appropriate.
Code size is not what the other commenter was referring to when they wrote "largest". There are of course other projects with more LOC than the Linux kernel.
