>>58732418Because GameFreak doesn't think it needs one right now, even though it'd be a prime recipient for one (a concept like "because it's already the common origin of all Pokemon, Mega Evolution has allowed Mew to assemble the mightiest powers of all of its descendants") from what we know of how Mega Evolution works, and especially the fact they gave TWO to Mewtwo.
As was pointed out earlier with Magearna, they can literally just invent a new Mega Evolution on the spot if they feel like it, so even though in-universe it would make logical sense to make a Mega Stone for every single fully evolved Pokemon based on this groundbreaking development, they just don't because that would be too many new designs, stat spreads, abilities, etc. to factor in.
