Why do atheists deny that they are just biorobots?

[Thumpalumpacus]

[...] but it seems to serve many very well as a springboard for various doctrines like naturalism, moral relativism, and fatalism.

I think those views are correlated with, but not caused by, atheism.

I know for myself that my views on the nonexistence of gods and my views on the relativity of morality, for instance, are both rooted in my epistemology of rational consideration of evidence and phenomena.