Emma Frost by Frank Quitely
The flippant answer to this question is “because you live in a comic book, and there’s always got to be an issue next month.”
The more difficult answer is because, as the great sage James Ellroy more or less put it, “closure is bullshit.” With every new person born, the business of teaching humans to be decent to each other starts over again.
(From New X-Men #126. Written by Grant Morrison, penciled by Frank Quitely.)