um god kicked them out of the garden over the fruit and adam and eve became mortal but no where in the bible does it say adam and eve refused to talk to god and even if they had each individual human could still make that choice and even if humans don't talk back a god could still introduce itself to everyone and give every one all the relevant information about itself
so again
why would there ever be anyone who did not know god if god is not man made?
So again...
Because of what happened in the garden of Eden, when man made the decision to separate himself from God's guidance...