There is no single frequency of light that will stimulate the different types of cones in the correct levels for you to perceive magenta, that only occurs if there is a mix of frequencies hitting your eyes that stimulate the blue and red cones a lot without stimulating the green cones, and as the green cone range lies somewhere in between the red and blue cone range that can't happen with a single frequency.

That I know. It's more a question of why we can't create light at arbitrary wavelengths, not why we can't create any given color using only a single wavelength.