Why are you replying as if you were a teen cringelord?
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.
But the words 'meant to' suggest that it is designed or intended to be read piecemeal over a lifetime, when in fact the history of what we call 'The Bible' makes it clear that this is not true.
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.
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.