Quick link for those who haven't read Fermi paradox et al. https://en.wikipedia.org/wiki/Great_Filter

Inline summary for the lazy: If we don't see intelligent life, and the universe is as old as it is, then it must be unlikely for life to persist to a stage where we could detect it. This unlikeliness can come in two forms: either it's unlikely to ever be created (hurray, we won!) or there's some future stage that's it's unlikely to pass (uh oh us).