I think this is also a likely cause, the odds that life even develops beyond the microscopic level, plus the fact that civilizations probably don't last that long combined with the scale of the cosmos and the light speed limitation means that it's unlikely that two civilizations even become aware of each other.

Still, I think it's a bit surprising that no signs of former life have been found so far in both Mars and Venus especially, and even elsewhere in our solar system like on certain moons.