My question is: Why don't all atheist consider themselves agnostic?
Because some atheists just don't care. Some of us can go entire months without the thought "I wonder if god really does exist" entering our mind. I know I do.
By your definitions I am both atheist AND agnostic in that I think the concept 'god' is simply unknowable; AND I actively deny the existence of this hypothetical unknown in the form of the anthropormorphisms claimed by every religion I have encountered.
I think the comparison of god to fairies, elves, and intestinal lemurs is apt; and I think that god, as defined by the bible, Koran, Torah, etc, IS falsifiable in that the god of Abraham did not do the things ascribed to him by the bible - e.g. stop the sun in the sky, flood the earth, etc.
Perhaps a better word, for me, would be something like 'untheist,' because I am not religious, and don't look at religion or worship in anything more than in political, social, and historical context.
That's why I don't understand a question like 'Why don't all atheist consider themselves agnostic?' Because my initial response is 'why would anyone care if they do or not?'