For the same reason chips are garbage. They impart a very monaural oak impression, it's like a monotone 'THIS IS OAK' flavor.
Take a look at a spiral and take a look at a cube. The spiral, like a chip, is going to have the same toast throughout. The cube is going to have a toast gradient more like a barrel would, so not only are you getting, say, med+ notes from the cube, but you're also getting fractions that you would get from medium, light and untoasted from the gradient; this, in my opinion and experience, imparts additional complexity and depth. That's why I recommend resting on them until they're neutral at 3-4 months rather than yanking at 4-6 weeks.
I've done side by side batches, and only two people (out of 15) failed a triangle test, and the mead rested on cubes was universally preferred among those who passed the triangle test.