This always makes me think. Cockroaches don’t bite, are not venomous, basically can’t do us any harm. I’m not afraid of wild dogs, spiders, tarantulas, scorpions, snakes, wolves, hyenas, wild hogs, bears (encountered them all) but I might actually scream if I get surprised by a cockroach.
I think this collective fear of cockroaches that we have cannot be without reason and must have an evolutionary reason.
As others have said: they’re nasty. There are a myriad of health risks associated with roaches in your home. They definitely do us harm, albeit not as directly or obviously as, say, ticks or mosquitoes.
I consider my fear of roaches to be entirely rational. They directly carry diseases, and they produce allergens. They exacerbate or even cause Asthma. Also, it’s never good to have decaying corpses of any type inside your walls, which is where they will end up if you have an infestation.
They will eat anything. They will go anywhere. They will thrive anywhere. I’ve seen wallpaper come right off the wall because roaches ate all the goddamn glue. You cannot get rid of them without a whole lot of poison (if even), which is itself a health risk. If I see a roach, I go to DEFCON 2. (DEFCON 1 is reserved for bedbugs.)