Meant by whom? The original authors of the Torah? The various prophets and Kings? The original authors of the New Testament? The Jewish scholars who compiled the Tanakh? The Christian scholars who combined the Septuagint and the New Testament into a single work? Pope Innocent I who ratified the choice of Gospels?
It's not a question of who decided that, but because it's a complex book. And that is also true to pretty much every other religious book in the world, where believers reach to their respective holy texts on a regular basis to study them and have a deeper understanding of their faith.
Your final sentence is simply not true. Many religions have central texts which are short, unified in style and content, written over one person's lifetime, and intended to be completely read and understood by followers with limited time and educational resources. For example, the Qu'ran.