Drupal used to suffer from a case of "not invented here" syndrome.
Prior to Drupal 8, all the code within Drupal (with the exception of jQuery, I believe) was written by Drupal Developers.
The PHP code was mostly procedural and didn't match how modern PHP code was being written in other projects.
There was no other third-party code and we had to rely on downloading zip files from Drupal.org, using drush dl or Drush Make - a Drupal-only tool for specifying and downloading the specified versions of Drupal and other modules you were using - to get the latest updates.
Drupal.org has its own Git hosting, testing infrastructure and issue queues.
Luckily, since Drupal 8's release in 2015, this has been changing.
