There isn’t a word for someone who doesn’t believe in Buddha. Or a word for someone who doesn’t believe in Bigfoot. Or a word for someone who doesn’t believe in elves. Or leprechauns. Or fairy’s. So why are those who don’t believe in God called Atheists?
Most men believe in God and logic dictates first cause and no Atheist has an answer for it. As far as Bigfoot and fairies go, there is little to no evidence that they exist and very few believe in them. Hence why they are non-starters.
Take the Holocaust of WWII. There are some who deny that ever happened and there is a word for that, “Holocaust Deniers”. That is because there is very good evidence that this happened and most believe it happened too. Hence the word “Atheist” also exists because most believe in God and there is good evidence for the existence of God, even though they will deny that. If the existence of God was a non-starter as Atheists often say, then the word Atheist would not need to exist.