Ask HN: Why are expert practitioners often so bad at teaching beginners?

This may be only my observation, but, I have observed this many times. A truly exceptionally expert practitioner doesn't always know how to teach a beginner the workings of his field. He knows his field inside out and are almost always self-taught in achieving their expertise. Still when someone asks how do they self learn (atleast start) they are clueless or point to obscure or very high-difficulty resources.

Is it because the area they practise in now, is at a much higher level of abstraction than what a beginner should do and they have lost touch? Or is there some other reason?

E.g: You ask an expert programmer for advice on how to learn to become a good programmer, they will almost always suggest mid tier resources.