Easy. Ancient human civilizations are proven to exist. Aliens aren't. Therefore, it's far less of a leap to postulate an advanced human civilization than an advanced alien one.
Much more importantly, ancient human civilizations existed in this side of the universe and knew about their own existence. Even if you had a ridiculously advanced civilisation with planet-moving tractor beams, it would be of no use for building the pyramids if they were a few galaxies away and oblivious to our existence.