This is how anti-reflective coatings work-- by "smoothing" the transition using materials that have intermediate refractive indices. E.g., flourite coatings on glass lenses.

Sound is transmitted by waves of compression and decompression in the medium. In the reflection scenario that's is ultimately molecules of air and water hitting the boundary and transmitting some kinetic energy into the other medium and bouncing back, to some degree. You can visualize it as marbles hitting each other.

It's much harder to do the same for light.

Thus, when light moves from a "faster" medium to a "slower" one, refractions and reflections result that are similar in character to acoustic reflections.

That's how I think of it anyway.

That said, it explains why glass is shiny, but not necessarily why it is reflective. At least to my understanding.

Empty space outside of planetary atmosphere is a void, there's "no" particles there (some Hydrogen and Helium and radiation).