Well, first problem is trying to have a detailed discussion about dentistry with the office staff. They typically know not much more than the general public about actual dentistry.
Pain does not necessarily mean infection, nor the need for a root canal. On the same note, lack of pain does not mean a lack of infection...dead nerves have no pain.
Your story sounds like a lot, a dentist who does not know the basics of what he does anymore and treats every pain as the need for a root canal. root canals are to dentistry as ADHD is to psychology, the most overdiagnosed treatment ever. You probably never needed a root canal by dentistry standards at all, just replacement of the filling after removing the infection.
Another story I keep hearing is:
My tooth was infected, needed a root canal. Decided to oil pull instead, and continued for X years. When I stopped oil pulling, the infection returned.
Does that sound like oil pulling cured the infection, or just kept it controlled? Don't get me wrong, I see OP as a great preventive treatment, but I have yet to see it as a curative treatment.
"So why is dentistry still focused on killing teeth instead of killing infections as the first course of action?"
It's not. Dentistry is by definition a preventive focused profession first. However, many unethical or unknowledgable dentists forget or ignore the basics. The key is to finding a good dentist, who still remembers what they learned beyond the drill and fill mentality.