Because 2D movies are way better at giving a sense of depth than 3d movies are. When I close an eye I can barely tell the difference. In a 3d movie everything is "ZOMG ITZ SOOO 3DEEEE!!!!!!" They try and give the impression that 2 people standing just next to each other are actually 1 person in front of you and one giant the size of a large galaxy several light years away. That's probably a big part of why they give you a headache.