I’m sure there’s no one single answer, but several that add up to accounting for the popularity of convenience products like this.
SR flour, while very common here…not everyone will have that. A lot of people don’t even bake any longer, so they wouldn’t have any flour, but if they have one flour, it’s AP/plain flour.
Also, convenience means ease, right? Bisquick has the directions to probably six or eight products right on the box, because it’s specifically made for biscuits, pancakes, cobblers, impossible pie, etc. It’s one box with everything on it. If I buy SR flour, now I’ve got to go track down a recipe for this, and a recipe for that…not a big deal when you like to cook/bake, but a right pain when you see the kitchen as nothing more than a room of chores. Pick up a box, look at the back, it’s all right there, six different things, in one spot.
There’s no doubt a good bit of psychology going on as well. Marketers are excellent at making consumers want to buy their products, that’s what they do, get you to somehow believe that using this product over that product will be easier and time-saving.
So…I just looked at Bisquick’s basic waffle instructions - four ingredients. I just looked at White Lily’s (a popular SR flour here) basic waffle ingredients - five. Not only that, but White Lily wants me to separate the eggs, whip the whites, and beat them in separately.
Now, I’m an experienced waffle maker (

), so I know that’s not necessary to make a decent waffle. I do that maybe half the time. But, if I’m not an experienced cook, and my only interest is to get some waffles down the necks of my screaming brat kids to shut them up for once and give me five

minutes of peace, I’m likely going to look at those to and say, “Bisquick it is. I’m not separating any damn eggs and I don’t even know what folding an egg white means!”
Remember, not everyone is as interested in cooking as we all are.
That’s definitely true, and I’ll always think of my generation as the generation of convenience food babies. I was lucky in that I wasn’t raised that way, but so many people of my generation were, so that’s the taste they developed, and that’s how they continue to eat.
That’s why I giggle a little at some of the folks I know who are a generation later than me, with their 85%-finished meal kits, bragging online about how they’re “cooking from scratch.” No, you’re sort of cooking from scratch, you’re finishing from scratch, it’s scratch-ish, but it’s not from scratch. I’m not knocking doing it, I’m just taking exception with the way it’s described.
Anyway, a little story to illustrate that. Several years ago, around 2010, my nephew came over to spend the night with his ultra-happening uncle (

), and at one point, I suggested we have some popcorn.
I got out a pan…some oil…some butter…some popcorn kernels…and proceeded to make popcorn.
He was stunned. He had no idea that popcorn could be made in any other way than by tossing a bag in a microwave and pressing start.
There are, I’m afraid, a whole lot of people who don’t even realize you can make better-than-frozen, better-than-delivery pizza at home easily, or that hot wings can be made at home…or Chinese takeout favorites, or that soup doesn’t have to just come from a can, that sort of thing.
Even when some people do cook things “from scratch,” more often than not, it’s jarred spaghetti sauce, or a premade pizza crust and jarred pizza sauce, or canned chili. Again, nothing wrong with those products, but sometimes, I think there’s a real disconnect between what we, as proper cooks and bakers, think of as homemade, and what a lot of other people out there think as homemade.