Obviously we're all introspecting here - but my guess is that there's some kind of cross talk in aphantasic brains between the conscious narrating semantic brain and the visual module. Such that default mode visualisation is impaired. It's specifically the loss of reflexive consciousness that allows visuals to emerge. Not sure if this is related, but I have pretty severe chronic insomnia, and I often wonder if this in part relates to the inability to drift off into imagery.
