I'm suggesting the circumstances are different. In each of your examples the have the knowledge that outside help was on the way, giving those people hope to persevere.
But in this situation, all forms of communication are gone. No help is coming and no knowledge of conditions in the rest of the world is available. Yes, some people will wait for help to arrive and starve, but a good percentage will realize the extent of the problem and go looking to save themselves.
Imagine the scenes from Katrina of the thousands of people walking out of the city that we got. Now imagine that those people never found "rescuers" just more and more refugees coming from other towns/cities. Food gets tight, it becomes a bloodbath.
Read Sterling's Dies the Fire
. It covers the situation pretty well.