Why are RGB tertiary colors so similar to RGB primary colors?
Likely a gamma correction issue. Tertiary colours are some permutation of (1, 0.5, 0) in RGB, right? So the issue arises because without applying gamma, 0.5 appears less than half as bright as 1, so the blended colour skews towards the primary (1,0,0) rather than the secondary (1,1,0). The image below has linear colour on top, gamma corrected on the bottom, you can see the gamma corrected colours appear much closer to a halfway tone.