The human brain doesn't have the capabilities of synthesizing the richness of life de novo. Dreams and hallucinations are transitory and ephemeral things. Longitudinal consistency makes the idea of such a long-term hallucination absurdly implausible.
We can only really think in terms of likelihoods. Why would a non-human super-organism, who had the mental capacity to simulate the external universe with such consistency and fidelity, dream up such a human life?
It's all a fun reasoning puzzle of course. We don't really fear that we're dreaming, we fear that we're awake and that this is what life is.