Why do Atheists compare God to fairy tales and a flying spaghetti monster?

No - of course we don't believe that a 'flying spaghetti monster' exists.  That's the whole point.  We believe that the probability of God existing is just as likely as a flying spaghetti monster existing.
Many people have faith in other gods, in fairies, in pixies, leprechauns, etc.  That does not, in any way, prove that they're real.

The existence of God is a philosophical posit rather than a religious one.  The philosophical argument that God exists is a simple one.  Religions tell us what God is like and whether anything is required of us.  Envisaging God as a bowl of pasta says a lot more about those who make the claim than about God.  Most of those on Y!A claiming to be atheists are naughty 12 year olds who have sadly grown up with insufficient parental discipline.  We should pray for them.

Have you ever had spaghetti alla Matriciana? They are made from many ingredients, and they are served on the table, they are not flying monsters!
So, do I notice your pathetic stray dog madness?
God is there, he reveals himself only to those who seek him, through prayers, atheists like you will see only on the day of their judgment!

Comparing God to fairy tales is to point out that there is no difference between the two in terms extraordinary claims being made with no evidence to support them.  Generally, we don't compare God to the FSM as it has a completely different usage, primarily to fight teaching of creationism as a science.

We all know God exists, your Indian ****, tell that to monkeys like you!
In the name of Yahoo, I find you guilty of atheist insult, reported!

Faith isn't evidence or proof. It should not treated like it is. 

Making up a silly deity to compare to what you believe in is the point here. That it's all made up and silly. 

The Bible and fairy tales are both filled with stories of magic (except that the Bible calls them miracles). The Bible even has a couple of talking animals, like fairy tales.

Ridicule is the last gasp of the defeated.

It's their way of explaining that you cannot prove something does not exist, when there is no evidence available to do it - like the existence of God. Having faith is OK, but it is not proof.

I like spaghetti. I wish there were more spaghetti monsters out there. I'd eat them up !!