I agree with you that any story or piece of text, which is generated by a robot, be as human-like as possible.
No human would write any story or text in exactly the same way twice. We would include the use of synonyms - both in terms of the variety of the vocabulary we use, as well as variations in sentence and paragraph structure and form.
However, only one software company owns the US patent of this feature.
Yseop (full disclosure: I work for them) is a natural language generating software based on artificial intelligence which writes - truly just like a human being. We are able to do this thanks to a unique, patented aspect of our technology which allows us to incorporate synonyms, both in terms of word choice and sentence structure on the text. Each person can specify the vocab words most appropriate to their industry and the type of output they want the robot to produce, and Yseop ensures the human-like nature of the text!
In our ideal world, readers wouldn't ask this question because they wouldn't notice the difference!