One possible reason why we haven't found any intelligent life in the cosmos the Great Filter argument, which says that very few civilizations make it to the advanced spacefaring stage.

How the 'Great Filter' could explain why we haven't found intelligent aliens

Source: space
Source Link: https://www.space.com/space-exploration/search-for-life/how-the-great-filter-could-explain-why-we-havent-found-intelligent-aliens