Why are there so few movies with the focus on the beginning of an apocalypse?

Particularly zombie apocalypse, but also more generally as well. Many seem to be about post-apocalypse, but the chaos and confusion at the beginning seems like such a fruitful idea. Yet that’s so rarely done/done well as far as I know for zombie movie/shows, except for shows like Dead Set UK. Is it because of the budget, or that such shows/movies cannot be milked forever like TWD?