Also because it's by far one of the most common things people get wrong, or could do better, when implementing alpha blending.

You can get 90% of the way to correct just by using square roots and squares. There's really no excuse.

To the contrary, non-gamma-correct compositing is very common, since so many programs (both historically and today) simply never bothered to do it correctly--but we get good results anyway. In most situations we wouldn't notice the difference, it usually takes a somewhat contrived example to illustrate it well. You can see that the example chosen blends between primary red and primary green, which is not a natural color combination. If you replaced one of the two colors with primary blue, even then the artifacts would be less apparent (because of blue's lower luminosity).

People currently shy away from making images where the current hacky method gives bad results (because nobody likes making ugly images). If implementations were fixed, they might not.