Except that even on an LCD the boundaries between individual pixels (RGB triplets) are imaginary. An RGB triplet forming an individual pixel is just a useful abstraction. But physically when looking at the screen an RGB triplet is not any special than a BRG triplet on the same display (ignoring the edges of the display and that some displays do have slightly larger gap between subpixels from different pixels).
And this is not just a theoretical concern. Exploiting that fact has practical applications for sharpness, aliasing, etc. Subpixel smoothing is based on that idea, and I think it's under-exploited in graphics in general because shading pipelines are, as far as I know, stuck addressing full pixels.
