The Drake equation was a famous way to parameterize our ignorance, but we still remain ignorant about the presence of alien life and alien intelligence. Hypothesized solutions have included:

that they’re there, but we aren’t listening properly,
that intelligent life self-destructs too quickly to maintain a technologically advanced state for very long,
that intelligent life is common but usually chooses isolation,
that Earth is purposely excluded,
that interstellar transmission or travel is too hard,
or that aliens are already here, but choose to remain hidden from us.

These proposed solutions usually leave out the most obvious option: that one of the three above steps are hard, and that when it comes to intelligent life in all the Universe, it’s just us.