Because "for real" deals with a different concept. "For real" is either used to determine if someone is being serious with their statement, or to agree with a statement.
"Are you being for real?" would be like saying "Are you being serious right now?", and "This place is boring" could be answered with "For real" to show agreement.
Now we're moving a bit into slang territory though. But this is why "you are not for real" isn't an acceptable translation.