Why can't movies stay loyal to the books they're based on?

this really bothered me with the potter flicks. like in episode 3 harry gets his broom early on, not at the fukn like 2 mins of the story

because they're completely different mediums, probably.

Because the author of the book isn't writing the movie, so it gets changed to fit the directors vision of what it should be not what it is. Also time, budget, all those things.