In the old days every blog on wp.com had Automattic’s own GA tracker inserted. So, it was clear then that you can’t have your own one because no site can have more than one GA tracker.
Today this seems to be an outdated reason as I can’t see any GA in the code. It would be interesting though to understand the current reasoning of why this community ‘idea’ (rather, feature request) is being ignored for a long time despite given amount of user requests.
Perhaps, a concise and reasonable answer to this question would stop creating similar threads and therefore even reduce groaning of some good fellows of mine ;-)