I think what Lucre mean is the future is not set in stone when it comes to the fate of humanity. But, from the way humanity is going, God can see the future outcome of it. If humanity acted differently, the outcome will be different. So, God sent prophets down to warn humanity the consequences of their action.
It like Marty McFly trying to change history because he knew the outcome.
But, I agree with you that the prophets kinda ineffective. They were not even consistent. Different prophet said different things. It's more effective if God use his booming voice to talk to humanity about the errors of their way. It will be better if he appears before them.
At least, people will believe in the existence of God then. And he should appear every century so that people won't say the books written about his appearance are fiction.
But, then, God won't be able to see human overcoming challenges and develope faith.