Why don't LCD printers use subpixels in color panels? The LCD resin printers I've looked at have pretty standard resolutions like for a smartphone and I understand they use the same technology. However, color LCD screens have three RGB sub-pixels for each color pixel. Check for example this magnified picture of an S-IPS LCD screen:

It seems like they could just omit the color filter and have three grayscale pixels for each color pixel.

3D printing just uses one color - UV. So why don't they have resolutions that are multiples of three of the usual resolutions?

All results about sub-pixels that I could find are about anti-aliasing, which is different (using the existing pixels better vs. having more pixels).
If what Thomas Sanladerer states is correct, the Mars 2 Pro (monochrome LCD, no color filter) has a layer time approximately 1/3 as long as the Mars/Pro printers (where color LCDs are used):

                
            
This would indicate that the light passing through the LCD, when the color filter is present, is 1/3. Put it in other words, only ONE subpixel is capable of transmitting UV light.
Consequently, using all of them would not improve resolution because the other subpixels are always opaque.