Film critics see a lot of movies, usually 4-8 per week, and have had the job for years. Having seen thousands of movies, film critics have a lot of things to compare movies to. They also have a job where their boss asks them to write about the merit of a film, as art.
Most audience members see 1 film a month. They aren't going to the movie as their job, to assess the art on display, they are going to the movies to escape from their job for some entertainment.
Since there are many, many more moviegoers than critics, a movie can make a lot of money by doing what moviegoers want to see. Even if it is derivative ( the basic concept of a sequel ) and not very original. These films make a lot of money, and studios make them to get the money they need for critically acclaimed movie production.
It's in large part an issue between evaluating a film as art versus entertainment. Along with this, there's a saying/stereotype/TV-trope that "true art is angsty," but most audiences go to movies for fun escapism.
What does good mean?
Critics look for movies that they believe are important. Americans want to be entertained. Critics regard art too much as something important. Popular art is often derided as kitche or low brow.
The big one is different goals. Typical movie goer wants a couple hours of entertainment. A critic has seen thousands of films, probably several that week and has a much broader experience to draw on. Taking a film like Transformers, it does really badly with critics because the writing is weak, the action and CGI are massively over done and lets not even get started on the statutory rape nonsense. Average movie goer got two hours of robots beating the shit out of each other with explosions and jokes, went home entertained.
Then you get the darker side. Ghostbusters kicked up a stir and started labeling anyone who said anything bad as sexist. That results in a lot of critics giving it a good rating for political reasons, which the audience doesn't have to care about.
There are a lot of different reasons for this, but one reason that stands out for me is the critics are generally evaluating the production and entertainment value of the movie, while the general audience is evaluating those things plus a host of other variables. Maybe they have some personal objection to one or more of the actors, or maybe they see some underlying political message that they don't like, or maybe the movie was just too different from the book it was based off of (even if the movie itself is still good).