Because mixing those RGB/CMYK?else levels are not the only factors affecting the actual color produced. You have environmental factors, human error, local tempe

Because mixing those RGB/CMYK?else levels are not the only factors affecting the actual color produced. You have environmental factors, human error, local temperature, media temperature, curing environment humidity, and on and on (and on). But if YOU have a pantone swatch and I have the same pantone swatch. We now have a reference that is the SAME color. I can tell you to keep adjusting your local parameters until the thing you're coloring (paper, fabric, plastic, rubber, metal) matches the swatch.