Just think of all the people who don't know how to change a tire, or some that might not even know how to put fuel into a car. I knew a girl in high school who didn't know how to put fuel in her own car. Her dad would drive it to the gas station and do it for her...
I think that's what owners manuals do is give you the basic information to do the basics on a car to keep it on the road or fix common issues without getting too technical.