Why primary colors are not used for printing?
The combination of RGB light creates white, while the combination of CMYK inks creates black. Therefore, it is physically impossible for the printing press to exactly reproduce colors as we see them on our monitors.
Why do printers use CMYK instead of RGB?
This is because there’s a wider spectrum of options with RGB color, meaning when you convert to CMYK, there’s a chance your printed colors won’t exactly match your original intentions. This is why some designers choose to design in CMYK: they can guarantee that the exact colors they’re using will be printable.