Have you checked that core Drupal and all the modules which have an upstream (different than the one who created the site) isn't just the same - but added to github to make it "simple" (ie. just duplicated code - nothing else) ?
Normally, Drupal developers are good at following the standard way of developing Drupal sites, which means NO core changes, and all changes done using hooks - in a module they write themselves, and in the theme itself.
So I don't think there's a lot of forking going on here - I actually highly doubt it.
I'm sure that's probably not the same for many other projects, but Drupal is actually a very well managed project with very sane development practicses IMHO. I particularly enjoy the fact, that new releases of same major version (f.ex. 6.0,6.1 etc.) just as RHEL does it, only contains severe bugfixes, and security updates - meaning you can actually upgrade rather safely, without getting featurechanges etc. They don't support each release for as many years, as RHEL does though - "only" until two new major releases has come out.
So Drupal is IMHO a very company friendly project, in that you get security upgrades ONLY - support for this for a long time - and security announcements as well.
PHP is in many ways not a well-designed language - but that does not mean Drupal isn't a great project or CMS Framework.