Let see ...
Their galactic empire rose and fell billions of years ago, whoever they were they're gone now.
Our atmosphere/pressure/heat was incompatible with their life form, and there are so many planets better suited to them, they never bothered to "terraform" our world.
They're currently living on Venus.
We are the remnant of a colony, there was a technological/societal fall tens of thousand of years ago.
They left a large black monolith in Africa.