Orks are many times larger, stronger, tougher, faster, fiercer, and more dangerous than the average human. They grow overnight out of the very air you breathe. When you kill an ork, ten more will take his place the next day. You can chop an ork's legs off or bisect him and he will still crawl to the fight and want nothing more than to destroy you and all you hold dear. The harder you fight against them and the more you kill, the more excited and frenzied they become. And for no discernible reason, they start gaining reinforcements as more and more orks pour into the system, attracted to the combat. Your only course of action is to redouble your attacks or risk being overrun, which only attracts more orks.
If you aren't being decimated by the towering boys and nobs, you're being dragged to your death by the grots. They send packs of beasts at you consisting entirely of sharp, jagged teeth mounted on a pair of legs, because nature said “Fuck you” and decided the only thing this creature needed was a way to kill you and a means to get there. Because they're orks, they decided to strap high-yield explosives to them anyways.
Once you see a single ork, you can guarantee more will be there in the future. And try as you might, you will never truly extinguish them entirely, so you'll have an ork problem in that sector for the rest of your mortal life. The only things that can cleanse an ork infestation are Tyranids and Exterminatus.
You don't fucking tell me orks aren't scary.