Because (unless you can show that you have right of abode in the country you are leaving) if you are refused entry at the destination you will be returned to the point of departure. If you do not have a valid passport then you will not be able to re-enter the country of departure and so you will get stuck in no-man's land at the airport.
The exception is when you have other documents showing a right of abode in the country you are leaving - hence the fact that I can leave HK using a valid HKID card without having to show my passport - if I am "bounced" by the destination country, the fact that I can leave HK on my ID card means that I can get back in on it.
Does my HK Birth Certificate count?