The thing that many don't realize when they are doing their "fan casting" is there are many factors to casting a film. Studios don't always have the ability to get anyone and everyone who "we" think they should cast. Actors schedules, actors pay demands, actors desire to work on other projects, actors contractual obligations with other studios, actors desires to only want to work with certain co-stars or directors, they were asked but turned it down, external issues preventing their casting, and more all prevent a studio from casting a particular actor in a role that "we" think they would be perfect for. Not to mention that sometimes they do an audition and it just doesn't meet the vision of the writer/director so they aren't cast in the role.
For an example brought up here, Jim Carrey was supposedly Disney's first choice for Genie before Will Smith was ever brought up, but Jim was busy at the time with a lawsuit and couldn't commit so had to turn it down.
So fan casting is fun, but there are many reasons why it doesn't happen the way "we" think it should.