What @Michael said, plus your retina has sensors for roughly red, roughly green, and roughly blue, and not for any other colors.
(BTW, the green sensors are more sensitive, so it takes less green to make the same brightness.)
When you see something yellow, it's in between red and green, so it excites both of them, and your visual cortex happens to call that combination yellow.
If your TV set turns on a red and a green pixel so close together that you can't tell they are separate, what does your brain call it? Yellow, because there's no way it can tell the difference between real yellow and red plus green.
Paints work by subtracting colors, not by adding them.
Blue subtracts red and green, and yellow subtracts blue.
When you mix blue and yellow paint, the color that is least-subtracted is green.
Colors you get by mixing paints tend to be muddy, as opposed to brilliant.