I'm pretty sure there's an element of games these days being marketed and designed (at the macro level) in such a way that they tend to get higher review scores. In other words, what I'm saying is that large publishers have learnt how to "tick all the boxes", to almost guarantee that they get a score in the 8-9 range. This means we get a bunch of highly rated but ultimately average games that competently fulfill the common criteria used to review games. Since the review score is such a key marketing tool and will affect the sales of the game, it's no wonder really.