Because it is fiction; entertainment, first and foremost. It's not intended to be a scientifically accurate treatise on exactly how it would be like in a post apocalyptic world short on people. Folks would be milling around all day gathering roots, scratching themselves, and wondering where to walk to next. Not exactly great entertainment.
How the various characters look and act is all about the elements of story-telling. People who are supposed to be likable in a story are generally made to be, um… likeable. That means the visuals are generally pleasant to look at so you can have that fully-immersive experience of being right there with them and completely empathizing with them as the main protagonists. You don't want your main characters to elicit a reaction like "gross, why do I care what happens to them at all?"
So they DO get squalid and unhygenic if the scene calls for it… but only enough to tell the story without becoming unpleasantly repulsive — unless that's what the storyline happens to call for at the time — but we generally leave that for the bad guys.