I'd guess it's because mixing colors in paint is subtractive—if you mix every color you get black. Whereas with light, mixing colors is additive—if you mix ever

I'd guess it's because mixing colors in paint is subtractive—if you mix every color you get black. Whereas with light, mixing colors is additive—if you mix every color you get white. In HSL, fully saturated colors have .5 brightness, and adding white or black changes the brightness but not the saturation. My guess is that works more similarly to paint than HSV does.