Can anyone tell me why companies let managers with absolutely NO training on how to select the best make hiring decisions?

This is more like a complaint, than a question. I dont have enough info to confirm your complaint. But, a short answer can be "because was that manager responsibility to take the decision and he assumed the full consequences/responsibility for it".

From my previous experience, when I started a recruitment process I did supervised closely HR department to establish the short listed for interviews accordingly with my requirements related to skills/candidate profile. Further more, I decided to hire and select from few candidates a new team member based on my feeling that was the right person.