As @benlings notes, the last reference to an actor can go away anywhere---in any task---so you are not guaranteed to be on the actor. You might be in synchronous code, so you cannot "hop" over to the actor. For global-actor-qualified classes, that's the end of the story: there is no way to get to the global actor without escaping self, which you aren't permitted to do.
For actor instances, you do know that you have a unique reference to the instance. For the native actors, we could call this isolated because we know that each actor instance is independently synchronized. However, this falls apart immediately when an actor has a custom executor for the same reasons that global actors don't work: the custom executor might be shared with other actors, and you cannot synchronize appropriately with them. Similar issues issues exist for initializers, which have a similar property of having a unique self (unless they escape it).
Doug