Why does the writer say that Frankenstein is not “a mere tale of spectres or enchantment”?

This can be found in the preface that Shelley wrote for the book. Mary Shelley says the story touches human passion and the identification of the human condition,

It was recommended by the novelty of the situations which it develops; and, however impossible as a physical fact, affords a point of view to the imagination for the delineating of human passions more comprehensive and commanding than any which the ordinary relations of existing events can yield.