Because, as the story of Philip and the Ethiopian eunuch reminds us, understanding Scripture isn’t something that can be done unless we have a guide:
Behold, a man of Ethiopia, a eunuch of great authority under Candace the queen of the Ethiopians, who had charge of all her treasury, and had come to Jerusalem to worship, was returning. And sitting in his chariot, he was reading Isaiah the prophet. Then the Spirit said to Philip, “Go near and overtake this chariot.” So Philip ran to him, and heard him reading the prophet Isaiah, and said, “Do you understand what you are reading?” And he said, “How can I, unless someone guides me?” (Acts 8:27-31).
If modern Christians understood that simple biblical truth adequately, two things would happen: 1) they would attend Bible study and 2) there would be fewer who would fall victim to false teachers and false teaching.