I like to think of this in terms of evolution.
In our universe, there aren't any trolls or vampires. So if things went sideways and suddenly there was a troll smashing cars, we'd freak out. The result would be YouTube videos galore, and some group of people would come and at least try to kill the troll. Things would escalate quite a lot, and it'd be a mess. In the end, humans work together to kill large threats as a matter of species survival, so one pesky troll would end up dead with hundreds of rounds of ammunition in it, and quite probably with it missing big chunks due to explosives of various sorts.
That's not the Dresden-verse.
There, humans have lived alongside the supernatural since forever. When we discovered fire, we hoped that it would keep away the things in the night. When we made swords, we hoped that they could kill the things in the night. But our best method of survival, on an evolutionary scale, has been to stay well away from where the monsters are, and stick our fingers in our ears, close our eyes, and shout, "LALALA! I don't seeeeeeeee yooooooooooooou!" whenever the monsters wander into our midsts.
This is a species survival trait. It's like how you don't need to learn to be afraid when a dog barks and rushes at you; our distant ancestors were eaten by large four legged things with big teeth, so we're naturally intimidated by them.
A troll is the same thing. It's an instinct for the folks in the Dresdenverse to pretend the troll isn't there as much as possible, because that's how you avoid it.
Of course, not everyone has the same instinctual reactions. Some people aren't afraid of dogs the first time one leaps toward them barking and furious. Some people are born wanting to do mad things like base jump. And some people are born without the instinct to run and hide from the troll.
In conclusion, most people have a primal urge to hide from that which goes bump and then pretend it hadn't happened. There might be some people who don't have that urge, but we call them, "Player characters."
I also tend to figure that intelligent supernatural beings with a motivation to not bring humanity down on their heads would fall back to behaviour previously considered antiquated. Ritual type behaviour, far from mortal eyes. So instead of fighting it out in public, they might call for some sort of duel in a ritual spot, a spot chosen for being out of the way.