Simplistic reasoning. Everyone has a different perspective, a hundred thousand individuals all labeling something terrible, does not represent the same resolution to another who believes it was great. Personal perspectives > mass labeling.
Nobody is telling you that you can't enjoy a movie though. I'm a part-time editorial writer for middle-sized film site and despite the proper terminology or standards or a degree I have- doesn't mean I don't appreciate the hell out of Spiderman 3. The Hobbit Series. The Nightmare on Elm street sequels...hell, Birdemic. They have value, but it doesn't mean they're great masterworks. Most people would and do feel collectively angered by seeing some of those movies I listed.
Seems like you're implying film criticism is pointless. People go to experts to know where not to waste their time, get entertainment reading them & not reward bad movies with their money. Film critics are NOT there to try and guess your opinion.
If critics who have seen previews dislike 'Assassin's Creed' it is because they're some of the biggest fans of film there are and want to get the word out to people who simply want a good time at the movies. The die-hards can and will still see it.
All you have to do it enter film forums or see comment threads on review articles to see that popular opinions are very agreed upon in film circles. LOTR? Good. Hobbit Films bad. Terminator 2, only good one. TDK is the greatest movie of all time, Suicide Squad is garbage. When the audience are so wide-scale agreed on these opinions it DOES serve a purpose to create content for your demographic and go off what you know the majority likes/dislikes. All reviews come with and really are an individuals' opinion attached anyway.
Meteorologists review weather. Sports commentators review sports events, I see little difference.