About legendary pokemon!!xd?

Im not s pokemon fab, but a have a question. In a movie like for example the one lugia appears, they later see him in the anime but the pokemon doesnt seem to remember ash like if the events in the movie never happend. And also .Why in the game you can catch legendary pokemon when in the anime they never do it????, I suppusoe thats one of the reasons why a dont like pokemon very much!!!XD