Because everything in Github is designed for growth: Easy to create a repo, very hard to delete it (a lot of scrolling, clicking, copy/pasting the full name of the repo, etc.) I mean "Deleting", not "Archiving".

MS and Github need their number to go up, not having people cleaning up their repos to avoid any loose ends.

I have hundreds of them, it took me a few hour to delete the unused ones. In a medium size org with thousands of them, it will take weeks for security to do a cleanup.