Seems a little like blind faith to me. Sure, we may not have anything to worry about but all those clinical trials were 6 months to a year. Who knows what kind of health risks inhibiting DHT for 15-20 years can cause. We just don't know. I'd certainly rather be as safe as possible and not touch type I and III and 5ar if we don't have to.
Unfortunately it's probably just not worth the money to optimize finasteride and re-market it when it's already "good enough" in most cases. So I doubt we'll ever see an efficient hair loss DHT inhibitor that only targets type II.