I dare say belief in mind-body duality may not be “accidental” in the least.
In the wider world, there are two sorts of “illusion”—accidental and contrived. There are cases where we get things wrong as the result of bad luck, and cases where we are the victims of deliberate trickery. When, for example, we see a stick in water as being bent, or when we think we are moving as the train beside us pulls away, it is a matter of bad luck. We are applying rules of inference in situations where our information is inaccurate or incomplete. But no one is trying to delude us.
When, however, we see a stage magician bending a metal spoon without touching it, or when we feel the table at a spiritualist seance lifting off the ground, it is a matter of intentional trickery. We may, again, be applying rules of inference in situations where our information is inaccurate or incomplete. But this time there is an illusionist who wants us to get it wrong.
Now, with belief in mind-body duality, which kind of illusion is it? The general view among materialist philosophers has always been that it is an illusion of the first kind, an honest—if regrettable—error. But how about if it is an illusion of the second kind, a deliberate trick!
Could it be? Only of course if there were to be an active agency behind it, playing the role of the illusionist. But who or what could possibly be doing this? And what interest could they have in encouraging individual humans to believe in a non-physical world?
The immediate, but unhelpful answer, might be that, since it is a case of self-delusion, the illusionist must lie within the subject’s own brain. The more interesting answer might be that, insofar as the brain is designed by genes, the illusionist is the subject’s own genes. But in that case, the ultimate answer must surely be that the illusionist is Nature herself, working through natural selection.