Well said Dennis and with most older furnaces, you would have to be able to read a schematic even if they did provide one for the electrical.
I've heard of doctors who avoid putting anything in writing so some malpractice lawyer can't pick through the paperwork and find a fault.
DIYers need to know their limitations and many don't. Building a house from start to finish isn't rocket science either but still beyond the capabilities of most diyers.
Come to think of it, having been to a few space museums, the basic principle behind a rocket engine (or a jet engine for that matter)isn't that difficult, but when you look at rocket/jet engines, the application of the principle is really rocket science and I'm not ready to build one yet.