My favorite theory: the von Neumann probes are already in the solar system. They've been continuously monitoring this system for a billion years. They could just be unwilling to make the first move toward contact with humans -- maybe we'll notice them, eventually, once we have better monitoring of small in-system objects. They could even be deliberately avoiding detection, like wildlife researchers who don't want animals to notice them. They haven't turned moons into copies of themselves for, basically, the same reason they haven't turned everything into paperclips -- that sort of unbounded-growth directive is useless and/or dangerous.

Note that I don't really believe this, but it does seem to be a resolution of the paradox that (like many others) we don't yet have enough evidence to reject.