Why can't an object's "__dict__" be stored in its __dict__?

Question

In this answer it is mentioned that an instance's "__dict__" attribute can't be stored directly in its __dict__. Why?

Furthermore: If instance.__dict__ gets transformed to type(instance).__dict__["__dict__"].__get__(instance), does this mean that instance.x gets transformed to type(instance).__dict__["__dict__"].__get__(instance)["x"] as well?

By the way: I have read the official descriptor documentation, but this still confuses me.

Thanks!