...stories. It's not intended to be 24/7 nudity. They would like it to be, these characters, but they're smart enough to know that, as with Mordor, you can't go around post-apoc America "naught but your skin," as Samwise Gamgee put it. It's a compromise between their personal preference for constant nudity and their harsh reality of living in a world without law, order, or a domicile of any kind other than their buses. They tend to go naked inside and in some locations outside, but they're able to discern the need to wear clothes in certain situations.
So, for any of you worried about their survival in the buff, don't fret. Imagine them putting on their clothes when the situation calls for it.