A lot of the atheists I've spoken to would actually answer "I see no reason to suppose so", rather than 'no' (although they might shorten their answer to 'no' for the sake of clarity, in some cases).
Atheism isn't (as far as I understand it) the state of having looked everywhere for God, and found him absent.
It's more like the state of considering the idea of the existence of God not nearly credible enough to bother with.
There are of course variation in atheists (and theists) but the simple fact is atheism is a negative on God existing and theism is a positive on God existing.