What's his motive for all of it? He cares passionately about what's true, in his own words. Not even just truths that have a demonstrated effect on people's lives, and more even than about what might make people happier overall. He thinks bedtime fairy tales are poisoning children's minds with their dangerous non-reality... he's like an SJ extremist. Plus, reading about him as a child, he comes across as much more of an Si-variety nerd than any other kind.
He employs a lot of metaphor in his writing but that doesn't necessarily prove a preference. I suspect that with his level and style of education that producing good imaginative and poetic writing is so rewarding to him partly because it contributes to a proud tradition of high quality literature that he knows his life was enriched by and that he can now give back to. And you can happily indulge in a function that isn't your first or second preference now and then. I have a thing about memorising maps.