Not to mention the odds of dreaming up an entire existence out of the blue, with no base reality for training data
Not requiring a world around the brain does not make it more likely, but less, as the probability of brains given that worlds exist multiplied by the probability of worlds is still much higher than the probability of brains regardless of the existence of worlds
> The Boltzmann brain gained new relevance around 2002, when some cosmologists started to become concerned that, in many theories about the universe, human brains are vastly more likely to arise from random fluctuations; this leads to the conclusion that, statistically, humans are likely to be wrong about their memories of the past and in fact be Boltzmann brains. When applied to more recent theories about the multiverse, Boltzmann brain arguments are part of the unsolved measure problem of cosmology.