The hard steps theory suggests human-level intelligence is a cosmic rarity, requiring improbable evolutionary leaps that may have occurred only once on Earth. However, some transitions may appear singular due to lost evidence or a priority effect, where the first successful group blocks competitors. While intelligence might still be uncommon, reasons could range from self-annihilation of civilizations to simple lack of interest in space exploration.
https://www.science.org/doi/10.1126/sciadv.ads5698#tab-contributors