First you are assuming parents, friends etc know how to dive. Most adults know how to DRIVE and I suppose could offer instruction to non drivers. My girlfriend is learning to drive at the minute and I chose not to teach her and let an instructor do it instead. I don't want her picking up my bad habits and the instructor knows what she needs to learn to pass the test. So much has changed since I started driving.
As for diving, the same applies. Not everyone knows an "experienced" diver. I certainly didn't when I started and wasn't about to let someone give me a "trust me I can teach you" course. Plus you gotta think about training equipment and first aid etc. While on land we would have access to a car to teach our drivers, would your friend/relative have access to 100% O2, defibrillator, first aid kits, compressors, cylinders, boats, buoys, etc.
There is a lot to be said about a structured course, that can go on to build on the foundation of the initial instruction, rather than "hmm what will I teach you today?" Stick with the instructors and use those that you feel comfortable and safe with.