God communicates to us through his creation and conscience (natural revelation) - Romans 1:20
Couldn't He just appear to us, yes... but at the same time that might interfere with our free will - He can not ravish, only woo - Isaiah 45:15
He also chosen a more suble of communication: Written language is a precise medium of communication is that can easily be duplicated
How are we suppose to know if this book is really from God?
Like a king's seal on a royal message, God reveals miracles only he can do to prove the message is from Him
A miracle happens rarely by supernatural causes
So if God wanted to confirm the word of God through Moses, Jesus, Paul or anyone else he uses miracles to do so
If we believe in the greatest miracle, creation, then every other miracle in the Bible is easy to believe