The notion that Earth is the only planet in the entire universe capable of sustaining life is not a view held by all atheists or scientists. In fact, many scientists and researchers consider the existence of extraterrestrial life to be a possibility, given the vast number of galaxies, stars, and planets in the universe.
The question of why we haven't yet discovered evidence of extraterrestrial life, despite the immense number of galaxies, is a subject of ongoing scientific inquiry. There are several hypotheses that attempt to address this question, such as the Fermi paradox, which explores the apparent contradiction between the high probability of extraterrestrial life and the lack of contact or evidence.
Some possible explanations for why we haven't encountered extraterrestrial life include the vast distances between star systems, the rarity of conditions suitable for life to emerge and evolve, the limitations of our current technology and observational capabilities, and the possibility that other forms of life may exist but are vastly different from what we recognize as life.
It's important to note that atheism, as a lack of belief in a deity or deities, does not inherently dictate a specific perspective on the existence or non-existence of extraterrestrial life. Atheists can have a range of views on this topic, just as individuals of various religious beliefs can hold different perspectives.
Ultimately, the question of whether Earth is the only planet to sustain life remains an open scientific inquiry, and future discoveries and advancements in our understanding of the universe may shed more light on this intriguing question.