It's because our concepts of a god, or the Judeo-Christian god, and the supernatural in general, work in such a way it cannot be 100% disproven ever, even if it doesn't exist, because there's always a argument to make about transcending nature, God staying in hiding, etc. As a results, most atheists like Dawkins argue that while a 100% knowledge is impossible, their evidence and arguments have make God extremely improbable in their view, around 99+% or something, although in practice their habits are round it of to 100%, not choosing to ponder over the 0,01 margin of doubt. Like I said earlier, by it's nature the supernatural can't be disproven fully even if it doesn't exists, so it's seems more like a formal doubt then any real doubt. If I'm in error, somebody can correct me on this. Some atheists are more formally agnostic then others, calling yourself a agnostic all the time, rejecting atheism, being the logical conclusion of it.
It's a little abstract, but we can't disprove we live in a computer program, or the entire universe is some being's imagination, and so forth.