Why? Well actually I KNOW why. Because most people don't like things they like/ enjoy being viewed critically. Most people can only have something be perfect, or with flaws they can rationalize away, instead of examining the possibility of maybe flaws with the core concepts, or that even aspects they adore, have flaws hard built into them, so acnowledging the product as a whole would also require assesing its flaws.
I don't even Take Reviewer Aggregates very seriously anymore, as they are still pretty colored by hype. Avatar Remains at 80+ Percent and I believe it was even higher before.
I find a reviewer who I think does a good in-depth job, and then stick with them. I ususally never feel comfortable purchasing a product if it only gets lavish praise because that often is tied in with hype.
Edit: Heck Prometheus stands at 72% (With a much higher initial score). Take quality, not quantity.