In Generation II, due to Shininess being determined by IVs and the fact that IVs are passed down through breeding, it is possible for an Egg bred from a Shiny Pokémon to have a chance as high as 1/64 of being Shiny itself, but only if the offspring is of the opposite gender as the Shiny parent. This is not possible in any later generations due to Shininess no longer being related to IVs.
Because it likely wasn't intentional. It's possible that this wasn't even known, but when shininess an IVs were made independent from each other, this mechanic ceased to work. Shiny Pokemon are supposed to be rare, not easy to get.