For me it's not even that, it's simply that The Brothers Karamazov and other novels like it have historicity and depth to them in a way that modern-style light popcorn novels never do. The hardness therefore is more incidental than the means to an end in itself.

And for what it's worth, there's plenty of modern-style "hard" novels that I've enjoyed immensely as well, including House of Leaves, Infinite Jest, Shogun, The Pillars of the Earth, and [pick any Neal Stephenson novel published this millennium].