The reason is simple. 'Atheists' could have chosen a self-identifier from any language during any era of human history, including coining their own brand new term. Instead they chose a term directly derived from Biblical Greek koine in the holy New Testament, which already had a longstanding traditional and perennial definition. Now they seem puzzled as to why 'Christians' continue to embrace that perennial definition over their brand new one.
The Greek word 'atheos' appears first in this scripture:
'...remember that you were at that time separate from Christ, excluded from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world.' (Ephesians 2:12).
The phrase translated 'without God in the world' includes this term.
An 'atheist' by its perennial definition is someone who is separated from Christ, excluded from His holy congregation, a 'foreigner' rather than a divine citizen because of their decision not to agree to the terms of the holy Covenant, existing without any hope of deliverance from judgment because they have abandoned God.
This is why Christians assert the term 'secularist' in place of an ongoing misuse and plagiarism of the term 'atheos.' We will never accept any assertion that God does not exist from ANY perception because of its abject absurdity. We do not even believe that it is truly possible for any person not to believe in God or gods, since the perennial definition for both God and gods also makes this impossible. Changing the definitions doesn't actually change anything at all.
The perennial definition of both God and 'gods,' ('elohim'), is a source of inspiration that affects human behavior even when they are not consciously aware of this influence. A 'deity' is any such source of influence which we elevate to the point where this influence is permitted to begin to take human life or the vital treasure which sustains a human life. When we decide not to pay this price, we can choose to switch our allegiances to a new source of inspiration and experience its influence instead. That is called 'repentance.' If we choose instead to pay the price, this is called 'worship.'
The God of the Bible is included as one of the well recognized and perennial sources of human inspiration. The modern assertion that these influences do not exist is laughable, especially considering that the 'atheists' making the claim are currently worshipping a 'spider' elohim with more passion than any worshippers in human history, (building 'world wide webs,' 'social networks,' a global infrastructure of interconnected energized lines and fine threads pulsing with intoxicating images, sounds, simulated realities, [even measured in 'bytes'], and all vigorously defended using 'public spin,' which includes the brand new definition of 'atheos').
'Professing to be wise, they became fools, and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures. Therefore God gave them over in the lusts of their hearts to impurity, so that their bodies would be dishonored among them. For they exchanged the truth of God for a lie, and worshiped and served the creature rather than the Creator, who is blessed forever. Amen.' (Romans 1:22-25).