Is there a reason why God decided to use men (and women) to spread his word?
I think you have not really understood how God speaks. He is spirit, so we recognise when He speaks to us, by the spirit - He is holy. The Holy Spirit testifies of the truth to all of us, yet we all wander in life, to prefer sinful ways, so we make it our way of life to resist the message of The Holy Spirit. Jesus said to Nicodemus "you study the scriptures diligently because you think that you have life in them, yet the scriptures testify of me and you refuse to come to me that you may have life!".
Only one man can be said to be The Word of God - that is Jesus Christ. Yet, The Word of God has also been with us all through time. John 1 shows the writer's view that the Word of God was the life that is observed, in the light of mankind. What is this "light"? Light is a pretty powerful, popular concept in Christianity - the opposite to darkness, it is where things are shown for what they are. The darkness on the other hand is where things are hidden. Ephesians 5:13 describes it like this:
Now all the things that are being exposed are made evident by the light, for everything that is being made evident is light.
.. Yet, you are asking something different. You are asking why you only get information about God from humans. I suggest, that those humans are making statements about God, because they have a faith in God. In that way, they are acting as witnesses toward you regarding their own knowledge of God. Yet, each one of those who has established that faith, has come to receive from God the living water. What those who haven't established faith have done, is taken a sip of water that comes from other men. Jesus said "whoever drinks the water that I give to them will never thirst, but rather it will well up to become a spring of everlasting life."
Isn't man prone to mistakes, bias, forgetfulness, exaggeration ...?
Yes! You should never go toward your destiny with God expecting to blame another person for what they told you about Him. Rather, test everything, and live blameless in His opinion of you.
Is this method not a recipe for confusion and ultimately a tainted message?
Yes, it is. Don't go after a blind faith, but ask Him for that living water.
FYI: experience shows, that our own shame prevents us from reaching out to Him. Yet, He grieves for us, and only desires more than anything, that we will ask for His help. Then no matter where we are, He will accept us, inform us what we must change immediately, and then go ahead with us through our whole life as He leads us to the green pasture.
Psalms 23 - I pray that you will receive this.