When we say its the word of God, we do not mean it was dictated, composed, translated, printed and commented by God. God inspired ancient authors about various thoughts regarding salvation and monotheism, namely in prophecies, but other than that, they were quite free to use whatever words or genres or style they wanted.
Its not written in a systematic manner like a systematic theology, thats why its so hard to extract such systematic theology from it. The European concept of systems has come with Aristotle, who lived later. And it was still just a beginning before it was developed more, into our current way of scientific and logical thinking.
The authors of the Bible (Old Testament) lived in the times and culture of Babylonia, old Persia etc. In those times, people used poetry, mythology, metaphoric stories, dramas... and its hard for us, who live today, to grasp such styles properly. Or to find easy systems in it.
On the top of that, there are common textual problems - multiple textual versions, multiple possible meanings of words and sentences, lost concepts we do not know about anymore but the ancient authors referred to etc.
Bible is quite simple in basics, but quite complicated in details. Thats why people can read it whole life and still discover new and new layers in it (and also why its so difficult to agree about those details between scholars, churches or individual Christians). Its quite a different reading compared to reading for example Wikipedia or a scientific paper.