Mythical Pokemon cannot be created from nothing. They require a vessel to form.
Mew has always been a tricky subject. It contains the DNA of every Pokemon. This makes other Pokemon incompatible. Humans are the only choice.
Humans have strong willpower. Humans will resist their body being taken over, for they would become immortal and remain a Pokemon forever.
Mews have adopted a unique way to counter this resistance: UDTF.
Humans will have their thoughts replaced with nothing but "wagwagwag" as a tail, making it significantly easier
to complete the transformation.