I liked a version of this (I believe) XKCD said once.
A perfectly valid response to the question, "why is the sky blue?" is simply, "because air is blue."
Yes, you can give a lot of explanation about refractive index and Rayleigh scattering. But all this does is provide details of how light interacts with air. And the same reason anything is the color that it is is due to the way it reflects, transmits, or absorbs light. You're citing real scientific principles, but you're ultimately just explaining why air is blue.