Here you are trying to define what an athiest is. That would be like a baptist,catholic,lutheran,morman,protestant,methodist,muslim, hindu bhudist etc, trying to define what a chritian is.You do not know, you can only speculate.Thats like a nonsmoker that has never smoked trying to understand why a smoker won’t quit.
Athiests define themselves, they profess a lack of belief in God. After that though, things get more complicated because they have a difficult time accepting the logical consequences of that lack of belief. The fact is, without a personal God who establishes absolute morality, its all simply personal choice.
One of the most common signs that a person is losing an argument is his attempt to obscure definitions.