Because the smaller they are the more difficult it is to see and find then....so many viruses and bacteria for ever mutate and return even more determined to keep on doing what nature let them evolve to :(
Because they are small. Because most of us think they are ugly. Because they have more legs than us and we can't figure why they need so many. Because some of them have more eyes than us. Because they can see in the dark and we can't. Because they lurk in places we take comfort in, like out beds and shoes. Because they appear out of nowhere. Because there are actually more of them than us. Because most of us don't understand their lifestyle. Because we know some of them are poisonous and don't really know what to do if we, or a loved one gets bitten.
Because, as a general rule, we are frightened of every and anything we don't understand.
This post was edited by Merlin at October 27, 2019 9:26 PM MDT
The question is too general. Personally, I am fascinated by them. On the other hand, I know someone who is afraid of butterflies. The other answers above are good.
According to the DSM-5, 6% of Americans have a phobia about some type of insect (or arachnid). Fear can develop from a belief that the insect is dangerous, or from a traumatic experience such as an allergic reaction to a bee sting.
I can imagine that if someone is OCD about cleanliness, they could develop a phobia about cockroaches because of where these bugs sometimes find some of their food.
I have a strong dislike for mosquitos because, in our area, a single bite can lead to Ross River Virus. The virus causes cold-like symptoms, which later lead to chronic fatigue and severe life-long arthritis. We have wire mesh on every window and door, and are very particular about not allowing guests to hover in the open doorway when arriving and leaving (which seems for some unknown reason to be a common tendency). I'm very sensitive to the sound of a mosquito and have become adept at swatting them when outdoors. I avoid being outside at dawn and dusk in hot, humid weather - or I wear a net over the exposed areas of my skin.
I also have a healthy respect for paralysis ticks - remove them a.s.a.p., and for spiders known to cause death, necrotic bites or severe pain. I know where these critters hang out and avoid the habitats.
Murray encephalitis is another mosquito-borne disease specific to Australia. Mozzies frankly terrify me, they are the most deadly creature on Earth. Spiders don't bother me, although I don't make friends with redbacks and no longer live in an area where funnel webs can be found. White tailed spiders are dangerous only if you're allergic to the fungus found on their fangs - the venom is harmless to vertebrates but the fungus is a fairly common allergen. I'm not, having been bitten an a few occasions without harm.
Hey, listen! I've seen all those old B-movies from the 60's and 70's where different insects mutate for some bizarre reason and grow extra large or extra smart and start going after the human population. So I wouldn't say I'm scared but I also ain't no fool. I just see myself as another average citizen informed by sloppy Saturday cinema. I got no time nor tolerance for the possibility of an insect uprising. Nuh-Uh! The bug in the house must die! In the wild they're just fine, but you can bet I'm keeping my eye on 'em.
Its a classic. Jon and I watched the "Tales From The Darkside" movie a few nights ago. LOL! Afterward we inevitably segued into a discussion of "Creepshow" as well. Fun fluff and awesome stuff!
And though he didn't get one, to me, actor E.G. Marshall deserved an Academy Award nomination for Best Supporting Actor for his role in "Creepshow" here -- he was stunning! He played "Upson Pratt" (he was terribly afraid of bugs) - - what a great name! :)
I think I've figured it out. You are like the "Mythbuster" guys! Your career is doing special effects for movies in Hollywood and beyond!! This is why you know so much about songs and movies!!! You are a behind the scenes SUPERSTAR!! :):) ;);)
Merlin 
