They have to do that because the books don't exactly follow a linear timeline. There's just so much shit going on and all if it matters. Martin has said that he deliberately wrote this series to be too big for the movies, they'll have to scale things down and rearrange events in order to cram it all into 10 episodes every year with each season ending on a banger. Luckily, they have lots of good starting and stopping points to choose from in the books.