GR doesn't explain why space time exists though. Quantum theory does, although there we have other problems such as explaining where the Born probabilities come from. At some point you simply stop and say "because that's how the universe works." Positing consciousness as the subjective experience of strongly causally interfering systems (my own theory, which I know doesn't exactly match Tengmark's but is closely related) doesn't tell you why information processing things like us have subjective experience at all. Maybe a future theory will. But even then there will be the question of why that model works the way it does.