Here's something to think about... The colour spectrum as seen in a rainbow is how it actually exists. And yet there's a colour missing. Whilst we can see violet and blue next to each other, there's no magenta. Because magenta doesn't exist. It's what's known as a non-spectral colour. Violet exists as wavelengths of light around 380 - 450nm in length, with blue coming next at 450 - 500nm.
So what is it when you see when you see magenta? Well, your eye sees colour through the cones in the retina. There's three types, known as L, M and S, for long, medium and short (representing the wavelengths of light they each see). So L cones are good at seeing reds, M cones green, and S cones blues. When you see yellow, your L and M cones are both sending signals to the brain as to how strongly they're being activated, and your brain knows that that signal means yellow. This is why pixels in screens emit blue, green and red light; by emitting light which matches what your eyes can perceive, you can stimulate the retina to see yellow (for example) through emitting an amount of red and green light that matches the same amount the retina would detect for actual yellow light. This is known as metamerism.
The stimulation of the different cones creates all the spectrum of colour you can perceive. Red to yellow to green to cyan to blue to violet are perceived from the extremes of the L cones through L and M, M cones alone, M and S together and S alone. It's also why violet colours tend to appear dimmer given the same environmental luminosity and material reflectance.
So if I shine a red light and a green light onto something, I see yellow, half-way between the two, because it stimulates my brain the same way. Equally if I shine a green and blue light, I get cyan. Shouldn't I see green if we mix red and blue?
You, right now, probably.
I see where you're going, and I commend you on your knowledge of physics. However, biology and psychology are weird. When you see red and blue light, your brain could in theory say "a-ha! Between these two is green, so I shall say we're seeing that". But there's a problem - unlike yellow or cyan, you actually have cones designed to see green, and those cones aren't firing. So your brain knows what it's seeing can't be seeing green.
As a result, it invents magenta, to explain how that stimulation can make sense. Magenta exists as your mind interpreting the light it's seeing. In fact, it's not just magenta it does this for. Everything between blue and red, known as the line of purples is your brain seeing light that doesn't have a wavelength. White, black and grey operate the same way - all the cones are stimulated equally, and your brain interprets that joint level of stimulation as something in the scale of complete darkness to pure white.
It's far from the only hack our brains use to explain the world around us, and exists as a particularly odd form of psychological explanation of the world. Our minds can't explain what they see, so they create magenta. Because the human mind is great at coming up with ways to make sense of the world, even when it's not exactly true.