The absence of evidence for aliens could be because they don't exist or because our sampling depth is inadequate to detect them... Fermi's question is valuable because it narrows the possibilities down to two: either aliens are not present near Earth, or our current search methods are insufficient.
Throughout the 20th century, astronomers faced this challenge while looking for planets orbiting other stars. Thousands of these exoplanets have since been found, thanks to dedicated surveys using bigger, more sensitive telescopes.
Optimists posited that the exoplanets were just out of observational reach; pessimists predicted that if they existed at all, exoplanets were well beyond the measure of any conceivable telescope.
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