First we’ll define science (from the Oxford dictionary):
the systematic study of the structure and behavior of the physical and natural world through observation, experimentation, and the testing of theories against the evidence obtained.
Now we’ll define atheism (from Wikipedia):
Atheism, in the broadest sense, is an absence of belief in the existence of deities. Less broadly, atheism is a rejection of the belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities.
The absence of evidence for any deity or supernatural power is the natural result you get when applying the scientific method to religion.
The key concept here is that science requires reproducible experiments. So let us assume that a deity or other supernatural entity exists. What consistently reproducible experiment can people perform which provides evidence for that hypothesis? There is none. (Though if you have one, I’m sure there would be a Nobel Prize with your name on it. Simply publish your results. You’ll be famous immediately.)
So what does science do when a proposed model does not yield experimental results? It rejects the model completely. A few examples of this include the luminiferous aether (a mechanism for light to travel through vacuum), phlogiston theory (an explanation for fire), or spontaneous generation (a theory for where life comes from).
Personal opinions or beliefs do not matter to science. Only reproducible experiments do.
Tl;dr - Atheism is science because there does not (and likely cannot) exist an experiment which supports the existence of God.