Taking a quick look at how color photography works is a good way to gain a better understanding of the differences between RGB and CMYK.
If we take a picture, the colors coming in through the lens are made up of light, not pigment. So, the input information (light) is RGB format rather than CMYK. However, when using film, light hits the film and causes the chemical emulsions on its surface to react. Those chemicals result in colors that are much closer to the pigments used in CMYK than to those found in RGB. When the film is developed (print film, not slides), the end result is a color negative of the image that came in through the lens.
If we examine a film negative we will see that when we take a picture of something magenta, ...