It's not because you don't lack belief in gods as rocks.
I do not? How so?
Someone saying gods are rocks have the wrong definition, and it's not unbelief or lack of belief that you reject in that situation but rather the definition. Lack of belief in gods mean that you lack of belief in the concepts of gods, not just any definition of gods. Some definitions of gods you would have to reject as proper definitions rather than "unbelieving" the concepts. You do believe rocks exist, don't you? So the god as rocks claim would be a rejection of the claim of rocks being gods rather than unbelief in rocks.
I thought the claim was not that we disbelief in rocks, but rather that rocks and babies are by default atheists?
Here's the deal.
1. "God" and "gods" are words.
2. They have meanings.
3. The meaning points to a concept.
An atheist lacking belief in god/gods lacks belief in all those things in different ways, but lacking belief in "god" as a word really means rejection of arbitrary definitions of the word, not lacking belief in a word (which would be ludicrous).
More like lacking belief in the need or usefulness of the concept, really.