Why do Athiests require 'proof' that God exists?

[Tempus]

So you're a weak athiest. That's fair enough. However as a weak athiest you must accept the possibility that God/God's could exist.

I'm not sure how you'd go about determining whether something is possible when it's an unknown. Weak atheism is another term for agnostic atheist. Atheism means someone lacks a belief in god(s). Agnosticism (as it relates to atheism) means one doesn't know whether god(s) exist or not. You don't need to accept something is possible or impossible in order not to know whether it exists. Indeed, you can't if you're an agnostic because you don't know whether any god(s) exist or have ever existed and hence can't know anything regarding possible existence or non-existence.