Its not even if there is a god (which I don't believe there is), but why I am certain that a god NEVER communicated with humans.
If a god ever did communicate with humans - these 3 things would be absolutely true.
1. It would be Indisputable. No one would doubt the message was from a deity.
2. It would be Unambiguous. Everyone would know exactly what was being said and asked. You would not have every third person interpret it in their own way.
3. It would be Globally Accepted.
Since none of these things apply to any holy writing - I know that no deity has communicated with humans.