The Prophets and their trusted scribes copied the holy writings.
Mere men didn't copy them, but very devoted religious leaders.
Like one would not dare miscopy or corrupt the Quran copies, same in Judaism and Christianity.
If the Holy Book was not authenticated by the Prophet himself, nor is in the same form as it was revealed to the Prophet, it can't be trusted as 100% the word of God, it's that simple.
One question how can someone who is unable to read and write validate and authenticate what has been written?