That is because they are pigs, sheep, cows, and so on. the only creature which would actually hatch from an egg is the chicken, which does actually produce eggs that can be "Hatched" by throwing them at something. it would create an egg in pokemon, because all pokemon hatch from eggs. Except for most legendaries. interestingly enough, Phione and arceus are exceptions to this. and Wasn't there a mewtwo egg event a while back?