Because that's not what's actually being postulated. What's being postulated is "you know the basic math of QM? well... Just take that math really seriously and avoid adding too many extra rules. a CONSEQUENCE of that is many worlds."

ie, "take the quantum amplitudes over configuration space and the linear update rule. Also keep the whole born statistics thing for now. Hopefully we'll be able to derive it from the rest. And that's it. Don't add any rules about the rest of the amplitude field going to zero or any other such nonsense. Just have all QM all the time"