It's important to note that atheism is not necessarily a claim of certainty that God does not exist. Atheism is simply the lack of belief in a god or gods. Different atheists may have different reasons for their lack of belief, but it doesn't necessarily entail absolute certainty about the nonexistence of God.
Atheists generally base their lack of belief on the absence of sufficient evidence or compelling arguments for the existence of a deity. They may view the concept of God as unsupported by empirical evidence or find the arguments put forth by religious traditions to be unconvincing. They often take a skeptical or agnostic position, which means they don't make absolute claims about the nonexistence of God but instead hold the position that the evidence and arguments presented in favor of God's existence are insufficient to justify belief.
It is true that atheism, like any belief system, requires a level of epistemic humility. It acknowledges the limits of human knowledge and the possibility of being open to new evidence or arguments. However, the absence of a conclusive explanation for the origin of the universe does not necessarily lead to the conclusion that a specific god or deity exists. Lack of knowledge about one topic does not automatically provide evidence for a specific alternative explanation.
Ultimately, the existence or nonexistence of God is a deeply philosophical and personal question, and individuals arrive at their beliefs or lack thereof based on a variety of factors, including personal experiences, reasoning, and evaluation of available evidence.