Well frankly, because there’s never been a single coherent concept of a god; one that doesn’t rely on blatantly fallacious reasoning, plain ignorance (natural or willful), or common wishful thinking; one that doesn’t just seem so contrived.
Sure you could just throw the vague term “God” out there (many do) but as soon as you attempt to define it enough to convince someone it’s a real thing, you will slip in bullshit. The whole venture (faith and apologetics) operates somewhere between pretending to know things you can’t know and avoiding knowing things you could.
Compare that to the vastly superior and convergent explanatory power of, and evidentiary support for, Metaphysical Naturalism, and the whole supernatural thing just seems so utterly archaic and fantastical.
Like a kid slowly discovering the truth about Santa Claus: it starts with nagging suspicions when things don’t add up, that gets covered with reinterpretations of the things you know about him (denial) with parental encouragement (theologians), then you learn too much about nature (reindeer don’t fly) to make it plausible anymore, and finally you find out that it’s centuries long parental conspiracy tradition co-opted by department stores. After that there’s really no going back, short of taking a ride in a flying sleigh to an elven toy factory at the North Pole. Atheism, for me, is a quotient of diving the value I place on explanatory evidence and truth by that of tradition and emotions.
Maybe I should ask you, why are you convinced that there is one?