It all goes back to fellowship, God is a relational God. He does what is impossible for man, but he also asks us to do all that we are able to do. God planted a garden, but he had Adam take care of it, because it was within the abilities God gave him to accomplish that. Man couldn't create himself, so God did( but then he gave them the ability to reproduce, so they did). Man couldn't flood the world, so God did (but man could build a boat, so man did). Man could write down what God spoke, so God let man do that. We are expected to do everthing we can to the best of our abilities, and God steps in to do the things we can't do, not the things we won't do.
The Bible wasn't written at one time, it was written by many men writting about the times they were living in and what God was doing at the time. If God would have written everything ahead of time, then man probably would have choose not to follow the historical events. If God would have written it all after Christ, there would be no Jewish nation for Christ to be born into. So we are left exactly as God had planed it. Each writer, inspired by the Spirit of God, wrote the events they lived, in the times they lived in.