the most common reason why people do not want to share their knowledge / experience because by sharing knowledge and experience then indirectly we also share secrets to others (depending on what is shared), some people are reluctant to share for fear of unrivaled and even got new competitors, like a chef who makes fried chicken super delicious, it is unlikely he will share the recipe to others just like that, because others can steal the recipe and make fried chicken similar or even much better (this is just an illustration).
