She's not in any of the new games because she has grown too powerful in between. It's obvious that Mima's power grows disproportionately fast, during her first appearance she's an early boss, next thing you see she's a final boss, and Touhou Soccer gives us the world-destroying TWILIGHT SPARK. What would happen were she to reappear now? No one could stand against her. She knows it, and she thinks the world is not worth her time.
I can't help but think of that whenever I see this image. Doesn't help that she's about the size of the Earth in it.
Sorry about the other comment. Screwed up the formatting and the post was too big to preview.
