Why does the chromaticity diagram look like that?

Purple totally exists, but isn’t a single wavelength of light. It is multiple wavelengths of light. Physical colors are all blends of wavelengths.

Displays are tricking the eye by showing three single colors that look like real color.

As a hue, magenta and purple shades do not physically exist in the electromagnetic spectrum. All hues on the gradient blue-green-yellow-red exist and can be generated by a single wavelength of radiation.

You can test this in physical reality with a prism, which will never show purple shades, because it is an extraspectral color that is made up in our minds.

Color can thus exist as pure in physical reality. However, our eyes can maybe not perceive colors purely, since our receptors overlap each other.

Colors are not single wavelengths. If you are redefining what color means, you should use a different word to reduce confusion. Maybe spectral color.

Secondary colors are colors. Notice that the hue on color wheel includes magenta and purple because it includes mixtures of the primary. Magenta and purple exist on electromagnetic spectrum but not as single wavelengths.

There are imaginary colors that are represented in color space but not by physical light spectrum. But purple and magenta are not imaginary. As can tell from the Roman emperors' clothing.

The discussion on this specific graph is fairly scientific and not about cultural color perception. I changed the wording to "hue" to be more clear. Magenta and purple absolutely do not exist on the electromagnetic spectrum as their own wavelengths. Every other hue of other colors we can perceive does.

"Primary" and "secondary" colors are not scientific terms, but cultural terms.

The evidence is right there in physical reality, a rainbow or a prism will not include magenta/purple, because the colors between red and blue are not part of the spectrum. It is an amazing thing that we can make up these shades in ur minds.

But I was wrong in my original comment, because the red-blue connection can also be done by making a color wheel, ie making a complete curve.

Edit: So let's make a comparison with sound, which is also waves, but not radiation. Every note of music corresponds to a pure sine wave. Just as every color hue corresponds to a pure electromagnetic wave. Except the purple and magentas, who do not occur like this.

Then you can say that most colors we see are mixed and reflected, which is true, just as most notes are not pure sine waves.