hey guys, i think i may have a theory on why the StudioPokemon become Eggs
my theory is, that after Genesect (#649) is just nothing but Egg values, kinda like in the Gen 4 Pokemon Games, where Victini's value was Egg and Snivy's value is Bad Egg (though they both had Sky Shaymin's cry in Platinum/HG/SS and no cry in D/P). the fact that they take the appearance of the StudioPokemon, i wouldn't know, probably it would have been possible to use your own Pokemon in PokeStudios, but they didn't go through with it. perhaps they would have had Pokedex info, who knows.
not to mention, all Wild StudioPokemon have Pound, a move that Eggs start with when encountered in the wild.