Singing well is easy. Learning how to sing well is hard.
Talking about teaching singing is easy. Learning how to teach is hard.
Developing the listening skills required to be a good voice teacher takes a long time. It requires hearing a lot of different people singing both freely and with various dysfunctions. There is no substitute for acute listening that leads to connecting sounds with vocal function. We can wish for and construct shortcuts, but they won’t hold.
Excellent voice teachers are excellent listeners and thinkers. It’s not magic. It’s skill. 90% of it is listening, in very specific ways.