You're misinterpreting that graphic
That colour space is showing hues of equal saturation and brightness. Brown is merely one of those hues with lower brightness.
In colours, RGB is certainly how light emitters and receptors work. However it's a bad representation of how we perceive colours after the brain has done its image processing. HSV or HSL are far better "natural" representations - and as you'll see from the Wikipedia graphic, all the hues tend to fade through brown on their way to black (zero "value" or "lightness").
And RGB sometimes isn't good enough
LED stage lights have been around for a while. The appeal is clear from a lighting point of view, because you can sweep between colours easily and instantly. And from an installation point of view, power consumption means you don't need kilowatts of power distribution, and you don't need lots of expensive faders (which are always electrically noisy and screw with your microphones). And it's better for performers, because those kilowatts mostly went into heating the stage up.
But after people had started using them, they found some obvious flaws. RGB LEDs aren't perfectly balanced, so it's hard to get a good clear white - and more than that, it's hard to accurately get a tinted white light. The tint is important, because a blue cast gives you the effect of moonlight, a yellow cast gives you sunlight, and orange gives you sunset. Following on from that too, it's also really hard to get good accurate shades of yellow from RGBs, and that's important because yellow is what gives us "warm" or "cold" light. Human eyes have high resolution to hues between red and green, so we can easily notice when something isn't quite right there.
So better stage lights aren't just RGB - they're RGBWA, with additional white and amber LEDs. That lets the lighting technician set the "lightness" in HSL space (between fully white and fully black) much more accurately with the white channel, and the amber channel gives much better control of the hue and saturation for anything between red and yellow.
LED TVs have picked up on this too. It's taken longer of course, because it's really technically difficult to make an LED TV screen, and just getting decent yields from RGB was hard enough to start with! So far I'm not aware of any which use full RGBWA, but RGBW screens are certainly available and advertise sharper whites and crisper colours.