You're right of course, but reading a review takes time and learning which reviewers have tastes aligned with your own takes even more time. The entire premise of Rotten Tomatoes is to give people a simple number they can read in a second.
Personally I disregard both RT and published reviews (I've never found a reviewer who aligns with me) and go off word-of-mouth from my circle of friends and family. But I think that doesn't work for everybody either.