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There are no rational causes for why we like the movies and music we like. Our preferences are based on emotions, and our emotions are based on subjetive associations\experiences\memories.
Reading many reviews from different people of said subject, although some may differ, would give you the majority consensus or the average opinion so that you can statistically have a higher chance of liking/disliking the subject.
This is not really relevant, though. The statement is that reviews have to be subjective. Whether reviews are pointless or not is a different discussion.
Finding a statistical opinion of a movie\music would in no way mean that the opinions are not subjective.
You can find statistics of how popular a music genre is, but that doesn't make the genre preference any more objective.
The point of a review is also to look at various objective qualities, such as overall quality (production of an album) and to compare the work itself to works similar in nature to it.
Quality of music and film is hard to define without any subjective values.
What is good sound? What is scary? What is unpredictable? What is interesting?
The expectations to quality are also known to increase with time, as the viewers\listeners are getting used to new effects.