Just because you were A good goalscorer doesn’t mean you will be a good coach. Temperament, attitude, teaching skills, understanding of the breadth of style etc
Forwards don’t need teaching “how” to score, they know by this stage of their career. What they need is the right drills, the right practice, the right routines, the right challenges and encouragement, someone to identify where their weak points are and coach them on improving it while honing their strengths etc
Keane showed how it’s possible to be a good scoring forward and a below-par coach, but then was it just the players he had? Without being there on the pitch it’s tough to say. It’s easy to say he was poor because our forwards were poor but they were already poor