Because in agnostic atheists view there is still no god. They are just smart enough to realize and admit that they don't know but that is a fact and the main focus of these terms are beliefs and their belief/view still is that there is no god.
As broad as atheism can be it all can still put into the groups of atheist and agnostic atheist. Anything else is either in the agnostic category or under the broad term of theism.
Atheism is the overall term. Gnostic Atheists and Agnostic Atheists fall within the spectrum of types of Atheism. Where you mess up is when you equate atheists with gnostic atheism. Most prominent atheists fall under the agnostic atheism label. Dawkins, Hitchens, de Grasse Tyson and so on... I'm not sure about the general public, but I'd wager there are some strong atheists, but most aren't.
From my experience, people who only accept the agnostic label generally are skeptics towards religion, and tend to be irreligious.... They don't have a god belief, which makes them atheists as well as agnostic. There is no such thing as a full on agnostic because atheism only requires not accepting theistic claims as true. If you don't accept a god belief, then you're an atheist.


