Well they do have a lot of things complete months in advance and just sit on the patches, testing them and fixing bugs for months. They have over 100 QA. When the schedule for releasing the patch comes up, they've fixed hundred or thousands of bugs, according to one of the panels they did where this process was explained.

Maybe they could instead release that in a very broken state months in advance to a test area but then it would spoil lots of things, and "surprise" is how SE markets the game and story is also valuable to them as a selling point so releasing it at the same time for everyone means some people aren't seeing it a lot earlier or trying to obtain data they shouldn't to spoil future events.