Here's reasons why calling the authorities might realistically go awry and tend to have people not do it. You don't have to overdo this, but even in our modern dayreal world people think twice before calling the cops in on everything for a lotvariety of reasons.
You may get arrested too. Did you do something illegal? Did you break and enter to find the ghouls? Then they arrest them and you too. Are you really not wanted for anything? If you live a very honest life, maybe calling the cops is a good idea, but if you (and all your friends/known associates) don't, maybe they're going to arrest you and go over all your loot trying to figure out where you got all that stuff. I don't know many adventuring parties that could stand in depth legal scrutiny.
Maybe they just don't like you or believe you over the other party. OK, if it's "monsters" it's one thing, but if you are a random drifter like most adventurers, and then come into town and insist the old man down the street is a monster, you're likely to get the "First Blood" treatment. (The first Rambo movie, I swear you kids nowadays...) Even in the modern day you don't want to call the cops a lot lest one time they decide you look too ethnic and decide to give you a good tasering one day. Authorities have lots of power to use physical force and incarcerate and deprive you of property with very little blowback if they're in the wrong and it doesn't always pay to hang around that. Do you sound and look crazy, like most adventurers? They're going to confiscate your weapons and maybe beat you.
Maybe they just don't care and don't help you. I have plenty of friends with horror stories about this from modern police, let alone medieval style. Or maybe they're just slow. Maybe they just get there after the sacrifice has happened. Whoops. They get their silver piece for the day, whatever.
Why The Pigs Won't Help You
Why is this something they need to do now instead of their other pressing activities, especially if they don't have a reason to 100% believe you? They go 8 hours later, and the bad guys have cleared out. "City Guard" don't exactly have CSI units so there's not necessarily proof. Is that human blood? Who knows. Bugger off.
Maybe they send a couple level 1 warriors to check out the problem and they get murdered bad. The reason adventurers take things into their own hands is that frequently they are the ones best equipped. The "civilian" excuse only lasts till you get a couple levels under your belt.
In realistic medieval settings, "City guard" is a myth. Professional police forces are a very modern invention. Earlier, you got the protection you could pay for, or not at all. Many places didn't have ubiquitous "cops" and it was every citizen's job to enforce the law! Read about English "tithings" and "hue and cry" laws - it's frankly more realistic medievally for "doing something about it" to legally be the job of those first on the scene and you'd even get in trouble for not doing something. Now, this is something youyou have to take into account when setting your campaign world up; if you've already set the expectation it's "COPS, Greyhawk City" then you've dug some of this hole for yourself.
Usually, you'd be looking for more personal reasons they might want to take things on themselves. "They kidnapped my kid, I can't trust this to the worthless guard!" "I want to revenge-kill that guy, best not tell the cops they'll want to take them alive!" etc. If you are providing a sufficiently high level of motivation, they'll be wanting to do it themselves.