For a couple reasons. For one, these aren't user reviews. They're written by people who are paid to think about and write about movies. I'm sure that there is a part of you that knows that for the most part this is not by random chance. They have this job because they're better at it than the average person.
The second reason is that to a large extent what makes a movie good is practically objective. There is such broad consensus about some things as to effectively take them out of the realm of "matter of opinion". Obviously if someone puts Catwoman, Iron Man, and Dark Knight next to each other, it would technically be a matter of opinion whether Catwoman is the best of that lot. But for all intents and purposes that's not the case. The latter two have story arcs that make sense, characters that behave and speak in believable ways (at least within their universes) and so on. If there were no objective criteria by which to evaluate film, film school would be pointless.
And, as much as you like to believe that you are the one special human, that's probably not true. You want it to be. We all do. But that's just your inner megalomaniac talking. If something is loved by 90% of people, whether film, food, or whatever, there is probably a much higher probability that you too will like it vs. something that is hated by 90% of people. Especially when supplemented by other info (genre, director, writer, actors, trailer, etc.). We're dealing with probabilities, of course, so sometimes you're gonna be in the 10% of haters. But that doesn't negate all the times that you fit in with the crowd, which is probably far more often than you're letting on. Something tells me that, like the critics, you'll give Dark Knight and Iron Man and Spider-man enthusiastic recommendations over Catwoman, Daredevil and Elektra.
And in any case, no matter the track record of statistical prediction, it's bound to be a more reliable indicator than the persuasiveness of a movie trailer that has been specifically designed to persuade you.