Probably because typing in Python is optional and kinda bolted-on after the fact, and you really need a good type system to get the most out of multimethods.

In particular, type declarations, as they were first introduced in python, did not have a run-time effect, and they very much have a run-time effect with multi methods.

(Dataclasses are kind of an exception to this rule, but one that feels far less fundamental than multi dispatch would).