I think it has a lot to do with trade secrets and the unwillingness to teach or give outright answers when someone has a question or is having difficulty. I tried the Moates BE system and no one would give me an answer it was always read this and then do the math yourself. If someone made a standalone or tuning system that the average person could use or understand(more input and tuning functions than a handheld tuner) w/o having to take lower level electronic engineering; I think it would take the market over. I have a background in electronics and some of the stuff was over my head.
I can tell you that F.A.S.T and Megasquirt are as you describe and usable by the average person, in my opinion. Megasquirt is all open source, both hardware and software, if you really want to get in the weeds with it. I expect most others keep that info as proprietary and don't share it.
I am definitely not an electronics engineer but I assembled megasquirt from a kit and got it running using the guides available at diyautotune.com and msextra.com. In the process I've learned a lot of stuff, like what a pull-up resistor is/does, what a voltage divider is/does, how a VR sensor works...
On one hand, I didn't need to learn any of that, I could simply "paint by numbers" and be an appliance operator as you put it, but the info is there to learn too.
In short, within reason, you can know as much or as little as you like and still be successful.
I expect the moates/binary editor route is likely one of the hardest, because as S_Mazza pointed out in another thread, it does not have manufacturer backing. A lot of the info isn't an exact science, it's a large collection of SWAGs. You can't get direct answers because there aren't ones to give. The level of understanding only goes so deep, and you'll get a lot of "just go through these motions, they work, trust me" kind of answers. If you want to know why, responses are going to be limited.