It’s due to having only 3 types of sensors. They combine as byproduct of capturing the photons, and while this is a part of the visual system indeed, it happens

It’s due to having only 3 types of sensors. They combine as byproduct of capturing the photons, and while this is a part of the visual system indeed, it happens even before the retina and brain get ahold of the signal. The signal itself is inherently representing an already-combined color.