Paint is a thing where (for example) RYB mixed together create black.
Light is a thing where RGB mixed together create white.
(One of those is *additive* and the other is *subtractive*, but I’m never sure which is which. I do know that RYB are the primary colors for paint and RGB are the primary colors of light.)
Because red, yellow, and blue aren’t the primary colours of light.
Our eyes detect basically three wavelengths of light: red, green (slightly more on the yellow side of green, but still green), and blue. All the light that we can see is caused by the three types of cells responding to those three colours of light. If the light is bright yellow, then both the red and the green detecting cells will react, since it’s close enough to their detection that they activate but don’t activate as strongly as if it was red light or green light.
Red (actually, magenta), yellow, and blue (actually cyan) are the primary *pigment* colours, because each absorbs a single colour of those three, so that mixing them produces a new colour. For example, magenta light absorbs green light but reflects red and blue light. Meanwhile, yellow absorbs blue light and reflects red and green light. If you mix yellow pigment and magenta pigment together, you end up with something that absorbs both blue and green light, which means it only reflects red light, so it appears red.