Is there a display technology that unites red, green and blue subpixels into a single pixel?
It would be possible to design a display with diffusing elements, each of which could receive illumination from three light sources.  It would be difficult, however, to have a diffusing element which would not send any light back toward its origin (wasting brightness), nor reflect any light that came from outside back toward the outside (creating an inferior "black level" or causing glare).  Additionally, is generally both easier and more effective to design all lights to have parallel light-vs-angle distributions than to try to aim each light toward the center of its pixel.  If lights are aimed toward the centers of their pixels, it will be necessary to either design the diffusers to counteract that effect or else tolerate significant color shifts with viewing angle.

For all of those reasons, even if it would be conceptually "nicer" for a 640x480 display to contain a 640x480 grid of three-color pixels than to have a 1920x480 grid where each pixel is red, green, or blue, the latter design offers substantial advantages which outweigh the fact that physical pixels don't quite match the logical definition.