Activities may actually run on completely different systems than where the workflow calls execute on. All "execute activity" is from a workflow perspective is telling Temporal server to execute an activity with a certain string name and serializable arguments. Everything else is sugar. So if you're gonna use a type caller-side to refer to the name and argument types, you can't instantiate it fully (its constructor may have side effects for when it really runs).

You can jump through a bunch of hoops like requiring interfaces which is what some frameworks do. But in our case, we just decided to make it easy to reference the method without invoking it or its instance.