That's because they have no valuable advice to offer.
They probably drew as kids and because of that have not had a moment in their life where they had to think about drawing analytically.
That's why people like Kim Jung Gi are so good at drawing yet so shit at teaching.
He literally did the "Just Draw" meme.
Not that "Just Draw" doesn't work but the efficiency of Just Drawing is terrible compared to concentrated and deliberate studying.
We need a teacher who is really analytical and also good at drawing, not just one of those.
And usually, this will come from someone who started drawing as an adult. Someone who took less than 10 years to get good at drawing rather than most artists who take 10+ years before they realize they need to learn the fundamentals.