Simply not true. If somebody makes a claim about the existence of something, it is perfectly reasonable to ask for evidence, regardless of what it is they claim exists.
Thinking of God as "something" is exactly the problem. God is not a thing. A yeti is a thing. A dog is a thing. The Loch Ness monster is a thing. A black hole is a thing. God is not a "thing", and therefore what evidence of 'everything and nothing' can there be offered, other than pointing to everything that is?
As I already said, it's up to those making the claim to define what they mean by 'god' (as well as provide evidence).
The very second the atheist says "Where's your proof of God", they have an idea already in the mind before the question is asked what God must be like, which is as I said, a "being", or an "entity" or some creature of sorts, separate and other to everything else. If you imagine God as that, then you imagine the rule of 'evidence' applies.
Same thing applies to those theists who think they can provide evidence for God's existence, and proceed to make arguments to support miracles and whatnot as evidence. All of that has a preconception of God as "other" to the world as their starting point.
It's quite easy to give evidence for air. So where is the evidence of 'god' and what, exactly do you mean by the word?
I was using a metaphor of what it is "like". For instance, a human generally does not think about the very thing they exist within (why I chose to say air). Same thing for a fish. A fish never questions if there is water. To them, that is just the environment and not a 'thing'. It's "like" that with God.
"In him we live and move and have our being". It just "Is" and isn't something outside what consists of everything they live within and gives them their very being within it. It's part of them, and they part of it. It's the unseen, yet not unexperienced.
If God is "all that is", then you have to get beyond that to see it as a "thing". You want evidence of God? Breathe. Exist. Be.