"An even more advanced civilization might view the entire physical world as a horribly primitive place, having long ago conquered their own biology and uploaded their brains to a virtual reality, eternal-life paradise. Living in the physical world of biology, mortality, wants, and needs might seem to them the way we view primitive ocean species living in the frigid, dark sea."
The author briefly covers this as a possibility, but given how relatively close we are to having the computing capacity to mirror and download human consciousness, this seems like the likely answer. Perhaps every civilization that develops the capable of interstellar travel, etc. just chose virtual.
A fun short story by Isaac Asimov entitled "The Last Question" talks about this.
http://www.multivax.com/last_question.html