Why are phobias described as an "irrational fear" towards something?

Because the fear cannot be reasoned with. So, if you had a fear of clowns, and I asked you why- you would probably not know the answer. There would be nothing for you to base your fear on. No incident with a rogue clown, or clown posse that scared you. You would not even understand where the fear comes from. That's the definition of "irrational."

Not all phobias are irrational- some are very rational, such as fear of flying or heights.

It is irrational because it does not make sense to the people who do not have the fear. Assuming the topic of phobias is viewed from a bias angle --> the idea that the person judging is normal <-- it would make sense to name it irrational because the person viewing the phobic does not understand why it is that the phobic is so afraid of a topic. For example, recently I read that anthophobia aka anthrophobia is the fear of flowers. Now, I personally love flowers. I cannot imagine why someone would be afraid of them. I can understand they may want to stay away because of allergies or some similar issue, but it is irrational to me for someone to have a panic attack when near a flower. All in all, this is what is meant by using the term "irrational".

I hope I was helpful :]

Because they are just that, irrational. Take a fear of spiders - you know in your head that they can't do anything to you and you could easily squash one, but you are still terrified. Or take a fear of elevators - of course you're afraid of the elevator dropping, but it's irrational to be so worried about that possibility that you avoid elevators altogether. The rate of things like that happening are vert slim. These phobias don't make any sense, and you know it, yet you're still afraid.

because they are illogical. Like I have a phobia or irrational fear of cockroaches. I start hyperventilating, sweating, and sometime even throw up when I see them. I know that they cannot hurt me but I just freak out. I can't even pick up and already dead one and throw it out with out getting out of control.

Sad but true. Even I know that this fear is irrational but I just can not get over my fear of them.

well most phobias are things that most people would not be afraid of but it is something that only a select few are scared of. this makes it irrational

Its irrational because a lot of phobias are of things that are silly...spiders, aluminum foil, cotton balls, mustard...etc etc. Its irrational to live your life afraid of a cotton ball....

All Phobias

I have a fear of touching cotton balls. I'd say that's irrational. They can't hurt me just from me touching them, but the feel of cottony substance makes my skin crawl.

Because the fear is out of proportion to the actual danger. For instance, fear of bugs. Yes, bugs are annoying and sometimes give a painful sting, but unless you are allergic, they generally are not going to kill you. Yet many people are deathly afraid of them. I'm not too crazy about them myself, LOL.

because most of these fears are unjustified. such as spiders, some can hurt you, but most cannot. but people are still terrified. it just means that most people do not find it scary or that it has no explainable reason that justifies fearing said thing