I don't believe the world is nearly as bad as you think it is. The problem with monsters and demons seem to be either sporadic and/or limited to certain spots in the world, where a demon happens to be living. Frieren & Co have seen a lot of it because they have been specifically looking for those spots while travelling.

Not to mention your view of the world is twisted by your own experience, which includes nothing but peace. If you go hundreds of years back in Europe, there simply was no peace. There were wars going on all the time. Sure, people didn't have the Internet or even the TV to get news from, but news about such big things spread with merchants, travellers, tax collectors, priests, and others. People just had no choice but to continue their lives the best they could, regardless. That shaped their minds. You don't see the "first world problems" line thrown around for nothing. If you live in certain parts of the world still today, you can't let the little things bother you anymore, as you never know if you are still alive a year from now.

In a fantasy world people as powerful as Frieren, Fern, and Stark are quite free to behave as they did in this episode. Few things can threaten their lives, plus they don't even have families or property to worry about. They are unfettered (expect for Frieren and Stark being fettered by Fern's mood swings), so they act accordingly.