Well, to make it clearer.
It's not like Atheists don't believe in anything. We believe many things.
Some beliefs may be a direct result or strongly correlated to atheism as far as you can call it an ism.
Basic atheistic worldview just means it is the default position on the existance of a god.
Being a sceptic means you are technically open to believe it if you are presented sufficient evidence.
Now there is also the position of strong atheism, or anti-theism, which I happen to hold and which overlaps with basic or weak atheism, and which actually is the believe that no gods exist.
Can I prove it to absolute certainty? No. But I also don't need to to reject your claim of a god unless you can provide sufficient evidence.
A good example is the bubblegummachine.
There is either an even number of bubblegums in it or an uneven number.
One of those must by definition be true.
Short of opening the machine and counting them doesn't give you a way to know and it would be silly to believe in either before that.
That and nothing more is the basic atheistic position.
Or the courtroom analogy:
In a trial a jury or a judge rule guilty or not guilty. They don't vote innocent.
The defendant can be innocent but that is irrelevant to a ruling of not guilty, because guilt has to be proven first no matter if he actually committed the crime or not.