Not because it is old. Because it survived.
A book read for two thousand years has been tested by every kind of reader: kings, prisoners, monks, merchants, students with an exam in the morning. What remains is what keeps working.
Read the Republic and you argue better, even with yourself. Read the Gospels, the Gita and the Guru Granth and you understand what billions of people were raised on. Read Smith and Newton and you know where the modern world came from.
That is how leaders are made. Slowly. On purpose. In company.