1) The फिल्में had to cut a lot out. Ginny and Luna's friendship is not central to the over-all plot of the books, so it was cut from the movies.
2) As mentioned, the book it told through Harry's eyes. Ginny and Luna aren't in his साल and Luna isn't even in his house so he doesn't see the two of them interact much. Since he doesn't see it, we don't see it.
3) As stated above, Luna is in Ravenclaw. Ginny is in Gryffindor. They don't have many opportunities to interact while in class (in the movies, Gryffindor's have all their classes with the Slytherin's. In the books, it's a little और varied.). That means the only times they can spend time together is during meals, maybe, possibly between classes and after classes, and on Hogsmead trips.
Thats because maybe they are best frndz but not mentioned in the book.You can understand their frndship if u go thru HBP.Ginny stopped some students from calling Luna 'Looney'.But mind you, Hermione and Ginny have a better frndship than Ginny and Luna.Maybe thats y, we don't see them together often.
