Historically movie critics judged a film based on its relative value. That's why you see a positive review on a cheesy rom-com and a negative review on a more ambitious flawed work of art. The rom-com review is viewed in the context of other rom-coms. For a positive review, it's saying "if you like rom-coms and their premise, this rom-com is good". I want it to be more prescriptive to what I will actually think when I watch the movie.
Most people use movie reviews to tell them whether they'll like a movie given its genre. That's why people say "I trust this person's reviews". They mean if the critic says the movie is worth watching and I decide to watch it, I will not be disappointed