First of all, most parts of the Bible does not require you to have the brain of Einstein in order to understand the message of the Bible, at least when you know how to read and interpret it. If you use the rule of letting the Bible interpret itself, and compare scripture with scripture when searching for a biblical truth, you are very likely to end up with very logical answers that do not contradict eachother.

When I first discovered this rule I was amazed to find out how often the most difficult passages suddenly makes sense. And this may in part be some of the reason why the Bible is so full of symbols, because unless we turn to God to have an understanding we won't get very far, it's one way of making us turn to Him for help and guidance and in that sense building a personal relationship to God.

Another reason for its complexity might also be that those who take time to study the book will soon discover that there has to be highly intelligent devine mind behind it.