In my opinion the main problem here is that such a high percentage of the population believe in a God or Gods. This makes being an "atheist" seem like it's something. Like it's a stance or a position, rather than just an absence of a certain belief.
No one calls people who don't believe in the boogeyman "aboogeymanists" or those who don't believe in levitation "atelekinesisists." That's because for the vast majority, the idea of the boogeyman or moving objects with your mind is immediately rejected. There doesn't seem to be a need to associate yourself with a lack of belief in these things because no one believes in these things. You're not an aboogeymanist because you say so, you're an aboogeymanist because you don't believe in the boogeyman, regardless of whether you say so or not.
But since so many people believe in God being an atheist seems like something more than being an aboogeymanist, when really it isn't anything more.
But don't mind me, I can argue about nonsense all day.