I don’t think it has been mentioned yet, but a big part could also be that the average visible normal on a 3-dimensional ground starts to shift significantly at grazing angles. The strength of that effect depends on how much height the material relative to the detail size has. I’ve done a quick test for it in Blender, comparing normal mapped planes with actual geometry. (And really this effect is in no way specific to only landscapes anyway, something like a wool sweater would display the same behavior):
A quick solution could be to bend the normal towards the camera, but that would require further testing.