If people can know God without the Bible, then why is there a rush to read the Bible? Salvation is a personal decision and if everyone knows God exists then why preach try to save people since only God can change people?
The answer is similar but not exactly the same as to why we have books on science and history and any other field of information. We stand on what others have learned instead of insisting that we will spend our time finding out all this on our own without any input from those who learned what we desire to know.
The Bible is written by men who knew God. They wrote, inspired by God, to communicate what decades of walking with Him taught them about Him, his ways, and man as well the record of what happened in the past, letters of encouragement and instruction and messages from God to man through prophets (in a nutshell.) We can either learn from them or go forward and make all the mistakes on our own never arriving at what they knew or we can benefit from what they learned (as in any other book) and read what they knew.
That is the dry answer. There is also the spiritual answer that God speaks to us through the Bible that cuts through our pride, on occassion. Pride is what prevents us from knowing God. It is was prevents men from believing there is a God at all. And God opposes the proud so there is a wall between us knowing God or knowing HIm better in which every stone is made up of our pride. And we, not He, must remove the stones. One way is to admit that others, the authors of the Bible at least, knew God better than we do so we admit this, humble ourselves, and read their words. It is through this admitting we DO NOT KNOW GOD well or at all that God is able to speak to us.
So we humble ourselves and read a book of the writings of men who we admit knew God better than we do and we admit we need help. That is why we cannot go it on our own. THe pride that wants to do it alone prevents Him from revealing HImself to us. As I said,
"God opposes the proud but gives grace to the humble. So humble yourselves under the hand of God and He will lift you up."