Because that is precisely and exactly what atheism IS: a LACK of belief in a god.
There is nothing else to say. This is the ONLY thing that all atheists have in common.
Apart from that ONE concept, we are as different and varied as everyone else. We are gay, straight, multi-coloured, educated, ignorant, rich, poor, honest or criminal, we are conservatives or communists, foodies and booklovers, beautiful or ugly. There is NOTHING else that we have in common. Nothing..