It's not that it's easier to imagine the end of the world than to imagine the end of capitalism. The end of the world is indeed the end of capitalism. For many people the only way to imagine the end of capitalism is to imagine the end of the world. Post-apocalyptic media usually doesn't depict people rebuilding after the apocalypse, so much as fighting over the scraps left over. The conceit is that without the current system everything descends into chaos. Supposedly, if we ditch capitalism we will look back upon the current times fondly as the antediluvian golden age.
This kind of fiction is what some call "nostalgia for the present." Unlike the supposed role of dystopian fiction as warning or satirical, the point of disaster/apocalypse porn is to make us feel like things aren't so bad after all. What little good exists in these world is not built or discovered, but salvaged from the present. As commodities without capitalist production, these things are an ephemeral reminder of the supposedly great past that has been lost by human folly. They will all disappear eventually, and the best the characters in these settings can do is hold onto these artifacts for as long as possible. You hardly ever see anyone building new things, beyond jury-rigging some scraps.
But why shouldn't people in the post-apocalypse build anything? Sure, in a disaster the supply chain will break down in the short term, but it's not like it's impossible to reconnect the raw materials and the processing centers. And with a sudden halt in manufacturing there would be huge piles of materials at all stages of production just lying around waiting to be used. It's not exactly logical, but do you think bourgeois film studios and other gatekeepers of mass media are going to allow a story where the workers just start using the means of production of their own accord? That would mean that the capitalist class are totally unnecessary!
The illogical nature of the standard post-apocalypse extends even farther than this, to a truly absurd degree. It's unusual in, for instance, a zombie apocalypse that somebody builds a wall or a fence to keep the zombies out. This would be a practical solution generally speaking, but where it does show up (as the writers have more recently become aware of the silliness), it's guaranteed to fail. Not just for "dramatic effect" (you can find other sources of drama besides humans failing to deal with the situation), but because in these kinds of stories rebuilding effectively simply isn't allowed. It's not part of the genre. The nature of the story is that humans are necessarily reduced to wanderers, hunter-gatherers killing each other and salvaging what little is left of the old world. When capitalist production stops, building new things stops too. And worse, previously built things often stop working. It's a very common trope in apocalyptic fiction where things break constantly. Even things that logically should not break. Even very shortly after society breaks down. There's no reason that cars would just stop working a couple of months into the apocalypse. There's no reason buildings would start falling apart within a couple of years. But the point of these stories is that if the world comes to a stop, it's all over. If these things didn't immediately start falling apart it would undermine the vision being created. If buildings just stood on their own (like they always did) and the world kept turning without the status quo continuing, that makes people ask questions. Just look at how people have responded to the corona virus impacting the economy. Reality is being clarified, illusions dispelled. In order for the ideology of dependence to be conveyed, a massive dose of unreality must be injected. And of course it does. The ideology is ridiculous, so too is its rationalization.
The sheer volume of this kind of fiction should raise eyebrows, and so should its uniformity. This is the only vision of the future we are supposed to have, other than things continuing essentially as they are. The only deviation we are supposed to imagine is disaster, and we are supposed to imagine it often. We are supposed to be too afraid of the alternative to seriously consider it. This is why not just post-apocalyptic fiction, but sci-fi, fantasy, anything not set in the present has developed the tendency to be relentlessly bleak. Where we experience hope it's supposed to be in some version of the present, even if it's a heightened one like the superhero movies or a near future where the basic model hasn't changed. If people feel hopeful looking at any alternative to this world, they might develop some desire to change things. Post-apocalyptic fiction takes this illogic all the way to its conclusion, saying that if things change everything will go to shit and you will long for the world as it is now. This is the kind of mentality used by domestic abusers to keep people in line too: make people fear leaving the situation and grow dependent upon and comfortable with their abuse.