Sometimes I agree with you (because it's annoying to me too), but other times I feel like that's accurate. The difference in quality between a book that's in the 75th percentile quality-wise among the books I've read and a book that's in the 25th percentile is not very large! I'd say, by and large, most books that get published are pretty good. Few of them are completely flawless or life-changing, and that's okay. 3-4 seems about right for 50% of the books I read.

If you combine that basic fact with a range of people with differing tastes, you get even more reversion to the mean, so just about every book has a 3-4 rating.

The same goes with beer. If you check the major beer rating sites, you'll see most beers end up with a 3-4 rating. Personally I think movies have a much wider range in quality, but you still see this effect somewhat with IMDb: a huge proportion of movies is in the 6-8 range.

You might think that the ratings would be more useful if what we got was a percentile rather than an absolute rating, and that might be right... or it might disguise the fact that I really would get close to the same amount of enjoyment out of a 3.5 as a 3.9, even though they're separated by 30 percentiles or whatever.