Why are the color printer inks magenta, cyan and yellow and not RGB (red, green, blue, the "primary colors")?

Additive vs subtractive color.

Additive – Adding light to black (no light)

Shine red, green, and blue spotlights on a wall – where they intersect:
Blue + Green = cyan
Red + Blue = magenta
Red + Green = yellow
Red + Blue + Green = white
No light = Black

Subtractive – Removing colors from white light, using pigments to absorb particular colors.

Shine a white light on a colored surface:
Cyan = White – Red (Cyan pigment soaks up red and reflects blue + green)
Magenta = White – Green
Yellow = White – Blue
Black = White – Red – Green – Blue

The Paper Mill Store – Color Theory: Additive and Subtractive Colors

The Paper Mill Store – CMYK and RGB Color: Which One Should You Use?

Green is not a Primary color. Green is a mix of Blue and Yellow.

Also printer inks are not light and don’t behave as light. Pigments such as paint are material substances and likewise have characteristics different from those of light.

Thanks, @Call_Me_Jay