If I were to guess, it might be because purple isn’t a wavelength of light, it’s like a glitch in how we perceive light with the two cones opposite to each other in the spectrum being stimulated at the same time without the middle one.
For any practical purposes in every day life, purple is a color, it just doesn’t exist outside our perception.
What’s the context on this photo? Why did he try to remove it from the internet?
Edit: thanks 👍