So what is the rational reason someone who isn't a subject matter expert (assuming that's what you mean by SME) to bet in prediction markets? The platforms appear to be marketed to the general public to be used casually as a way to make quick money.

Someone who is a "subject matter expert" (and given how specific many of these predictions are, being an expert usually means you are an insider with non-public information) can indeed make money as long as a bunch of idiots with worse information place bets thinking they know the outcome. This is considered to be a crime when done with other securities.