Perhaps they are an expression of frustration and doubt seeded in a populace that has come to believe that the society they live in is too fragile and there is an inevitable collapse.
Or maybe this genre reflects the primeval in us as homo-sapiens, that through these movies we live a vicarious life, one less ordinary, far removed from the mundane nature of our lives.
It could be that we have an instinctive need for violence and struggle, for our species has fought a monumental series of battles to achieve its place in the world.
Of course it could be something as simple as access. Film equipment is inexpensive compared to a decade ago. The secondary market is flooded with it. You can learn the basics of the trade at school. And apocalyptic movies are cheap to make as you just need old, dilapidated materials and sets.
Then there is the inevitable question driven by the central core of Darwinism: competition. What would I do?
There is probably no one answer, there are probably multiple answers and multiple reasons. That's the way it generally is. Seeking a 'reason' is more often than not, a fool's errand. Better to look at a bigger picture.