Why

I really wonder sometimes why people are afraid of things that pose NO threat at all... spiders i guess i could kinda understand.. i mean they do bite and some are venomous but why are some many people afraid of clowns? they're just people in make-up... not very scary, a little weird but not scary.. and there is actually a scientific name for fear of peanut butter getting stuck to the roof of your mouth.... it doesn't get much more ridiculous than than, how can you fear something if you realize the fear is totally irrational?