A lot of them aren't planning for the end of the world, just for "something bad" of indeterminate but finite length that's longer than a lot of people think reasonable.

It's a spectrum, with routine "emergency preparedness" on one end, and "self sufficient lifetime bunker filled with reusable water and canned food" on the other.

It's normal for people to have a flashlight, a few days worth of shelf stable food, a first aid kit and a couple of tarps. It doesn't even need to be intentional, it's just normal, but it still forms a basic emergency kit.
A rational response to a normal risk.

A lot of the more extreme peppers who aren't radical are in more rural areas, where something like a tornado could actually knock out power for a week or more.
A rational response to an uncommon, but real risk.

Others just have a disproportionate estimation if the risk of something like Katrina or the 2003 blackout happening, that can knock utilities out for a protracted period of time, or some esoteric and unlikely beliefs about civil unrest.
A rational response to an uncommon, unlikely risk.

At the far end you have people who want to survive the literal end of the world. I don't necessarily get why you would want to survive for a bleak and empty life either.
An irrational response to an unprecedented, infinitesimal risk.