One reason why is that Google Analytics is not being limited to giving the web site owner traffic information, the analytics are also being used by Google to collect and correlate larger traffic patterns, as well as track individual users across multiple sites. These are things Google gets to see, but the neither the site owners nor the users have access to. What Google does exactly with this information is not entirely known outside of Google, though it's certainly used at least to improve personalized advertising, which many people feel is a privacy concern.
