Well, you have to ask yourself why you're running a post-apocalypse setting.
You could run any kind of story or theme - a romance! Courtly intrigue! Mystery! Horror! in any setting. But why are you picking that setting, what does it reinforce?
Post-apocalyptic settings are about survival, demonstrating the folly of humanity, etc. Those are the tropes that kind of setting evokes. So choosing a theme not aligned with that setting begs the question of why you'd choose that setting as opposed to another. Trying to fit random themes into arbitrary settings can be fun I guess, but in general I'd start with the story and then choose a setting that actually supports it with its tropes.
To me it's like asking "how come whenever I play World of Darkness games we end up covering a lot of the same ground?" Well... Of course you do. That setting is superheroes + angst so stories you set there are going to rehash superheroes + angst at some level, unless you put in a lot of work to stay away from it. "Let's all play happy kids on a playground in Vampire! But no one will be a vampire and we won't meet any vampires." "...What?"