Genetics are a fickle thing, seeing as mew cannot breed with other current pokemon (and of course, since legends can't breed in games, two mews cannot breed) it shows that despite being somehow related, they're still genetically diverse enough to the point of being incompatible.
Therein lies the unusual thing, because entirely different species have the ability to breed in real life (tiger and lion, creating the Liger for example). So it stands to question, just how many eons of evolution have taken place to make them that genetically different?
Though if Pokemon were to follow how real life genetics go. Any hybrid of Mew and other pokemon would be sterile and be unable to breed.