For those wondering not from the US: it's because of world war 2. An entire generation of kids who grew up during WWII rationing, where pre-packaged mixes were a lot easier to find than straight sugar and a lot more stable and recipes almost always are based on what you can have on hand. Not to mention boxed cake mixes are absolute marvels of chemistry, like there really it a big difference over just self-rising flou…
A story I heard: the boxes of cake mix initially didn’t require eggs. Apparently housewives at the time hated them, because it made them feel sort of worthless, so the recipe was re-engineered to require a minimal amount of fresh ingredients; then they were a huge hit. [edit] oh, this story is repeated in a Youtube vid someone else here posted: https://youtu.be/CZDFwqHkPec?feature=shared
They always seem to turn out the same no matter how you cook them, yet the back of the box insists that you need to poke several holes in the plastic and stir it halfway through and let it cool for several minutes.
I'm convinced that they only add those steps to make people feel less guilty about heating up premade meals.