My old favorite, from Wait But Why:
An even more advanced civilization might view the entire physical world as a horribly primitive place, having long ago conquered their own biology and uploaded their brains to a virtual reality, eternal-life paradise. Living in the physical world of biology, mortality, wants, and needs might seem to them the way we view primitive ocean species living in the frigid, dark sea. FYI, thinking about another life form having bested mortality makes me incredibly jealous and upset.
My new favorite, from Terry Bisson:
"That's ridiculous. How can meat make a machine? You're asking me to believe in sentient meat." "I'm not asking you, I'm telling you. These creatures are the only sentient race in that sector and they're made out of meat."