This was on my mind as well, but I think the semantic guarantees of Codable are not quite strong enough for what we'd want ActorSendable to do. All Codable really tells us is that a type "can convert itself into and out of an external representation," but the behavior of that type after conversion to/from the external representation is left completely unspecified.
It would be perfectly valid to conform to Codable a class which is intended for use as a singleton and reads/writes lots of global data, on the assumption that Codable is only used for persistence between program launches. However, such a conformance is obviously unsuitable for ActorSendable, since two separate instances would be referencing the same global data.