If Joseph Smith had previously written the Book of Mormon before dictating it to his scribes, then he would have needed to find a lot of spare time outside of his responsibilities to the family.
Scholar Robert A. Rees said:
What we find in the historical record is that the hardscrabble life of the Smith family in general and of Joseph in particular seems to have left little space or leisure for the kind of thinking and writing necessary to produce a manuscript of the length and complexity of the Book of Mormon. Before Moroni’s first visit in 1823 and Joseph’s acquisition of the plates in 1827, Joseph was preoccupied with the family’s declining fortunes, working the family farm and hiring himself out as a laborer, as, in his own words, “it required the exertions of all that were able to render any assistance for the support of the Family.” Thus, the idea that Joseph had time to read broadly, undertake research, construct various drafts, and work out the plot, characters, settings, various points of view, and multiple rhetorical styles that constitute the five-hundred-plus page narrative of the Book of Mormon is simply incredible (in its original Latin sense of “not worthy of belief”).– Robert A. Rees – Joseph Smith, the Book of Mormon, and the American Renaissance: An Update
It is unlikely that Joseph would have had the luxury of so much leisure time away from his family responsibilities to research and write an entire book the length and complexity of the Book of Mormon.
Even if Joseph did find the necessary time to write the Book of Mormon, why would a religious boy choose to spend his time writing a fraudulent book of scripture?