When I was younger and despised the world, PA settings in games just felt right.
*Shrugs*
Since then the aesthetic simply pleases me. Especially when the story goes along the lines of "Everything was destroyed but people are keeping strong and making the best with what they have"
Empowering notions of human strength, etc.
You know, I think that's one of the other great elements of the genre -- its easy to accuse it of nihilism (the world is gonna end!) but the genre itself, with a few exceptions (The Road, I'm looking at you), is generally quite optimistic. Road Warrior and Beyond Thunderdome are both told from the perspective of survivors looking back and remembering the legend who saved them, and almost all the Mad Max knockoffs are built around this assumption that humanity will rebound.
Not to get political, but I think we're back in a cultural moment much like the one that gave birth to the genre originally -- this time the concern is less thermonuclear annihilation, and more catastrophic environmental collapse, but the end result is the same: civilization grinds to a halt and it all becomes deserts. And while it's easy to think of post-apocalyptic fiction as a gloomy warning, it can also be a cheery promise that no matter how bad things get, people will survive and find ways to thrive.