Why theano UFOs

It seems possible that the great filter is mostly a matter of time and
distance. A spacefaring civilization has no particular claim on
lasting a million years, let alone a hundred million. There's time for
species to have evolved, but there's also time for species to have
evolved into dead ends where they were too specialized to meet the next
hurdle.

Maybe the great filter is a dilemma. Species that maintain mixing
across their entire range evolve as one species, and come to a dead end
one way or another; species that don't maintain mixing across their
entire range don't spread enough across the lightyears to keep up with
the extinction of local populations every few thousand years.

Or maybe different species evolve, make contact, and annihilate each
other after periods of peaceful coexistence averaging only 10,000 years.