because it's from the perspective of the people on the planet... they never know exactly what happened so neither do we. you can study and intuit what happens post modernly like people who watch the matrix did but you're never gonna get one decisive answer.
doesn't that make for more interesting discussion?
it was the rapture. it was aliens. we all live in a cave, life is nothing but an illusion, and the people who all disappeared never existed to begin with. all plausible.