Why is the purple from the three primary colors not pure purple?

Because the high frequency band that human eyes can perceive is a kind of bluish purple, it is called violet in English, and its wavelength is about 380nm~420nm. Electromagnetic waves with frequencies higher than violet are called purple in English, that is, electromagnetic waves less than 380nm are imperceptible to human eyes, that is, they belong to invisible light band.

The reason why you can still see purple is because the color wheel is close to red, which is supplemented by the human brain, that is, the color perception obtained by mixing R+B, so there is a knowledge point. The light with the same color can be a mixed color or a pure color, such as yellow, a light wave with a specific wavelength (about 580nm) or a mixture of R+G, resulting in a yellow color sense. And purple can only be produced by mixing.