I chalk the need for everything to be literally real - the tendency of soulbonders to cling to notions that their soulbond is real in a parallel dimension or that they are full-on multiples - to a general trend in our culture of believing one must be a self-contained singular entity for your existence to be justified, and believing that one can only interact with a facet of the mind as 'other' if they are a self-contained singular entity. From what I've seen the multiple community is full of this idea, which is rather sad;

The multiple community is full of the idea that we're self-contained singular entities, aka people, and that's sad?

Maybe I misread you here.