Quoting Dave Henry

Most game worlds have trouble explaining why anyone tolerates that eccentric, heavily-armed, trouble-causing group of adventurers who wanders around poking their noses into things all the time.

It's really quite simple. Monsters exist.

Your average citizen, knowing that monsters actually exist, upon hearing the news that a huge horrendous monster is attacking the village from the North has the following (sane) reaction:

The Adventurer, on the other hand, upon hearing the same news, has the following reaction:

As you can see, Adventurers are clearly insane.

However, when they actually defeat the monster and save the town, well, they're awfully nice to have around.

Typical reaction to Adventurers is, at best, mixed. As long as they're not actively terrorizing the populace, well, it's kinda nice to know that they're around, just in case -- but you wouldn't want your sister marrying one.