Atheism is based on faith
Apologists often claim that not believing in a god requires just as much faith as belief does, if not more. Norman Geisler expressed this argument in the title of his book, I Don't Have Enough Faith to Be an Atheist.
"To be an Atheist one would have to be omniscient knowing all things having a perfect knowledge of the universe, to say they absolutely know God does not exist. For one to do this they would have to personally inspected all places in the present known universe and in all time, having explored everywhere seen and unseen."
Theists treat belief in God as a default belief, and they will often back this up with some variation of the argument from design.
In addition, asserting that atheists claim to "know" that there is no God is based on a misunderstanding of the word atheist. See the article on atheist vs. agnostic.
Since theists make a positive claim which is extraordinary in nature, the burden of proof is on the theist to prove that there is a God. Most theists do not "believe in" leprechauns, yet they would not consider a request to prove the non-existence of leprechauns to be reasonable. There is no reason why anyone should believe in leprechauns or God without positive evidence.