The reasoning behind this is usually that several of the workbook lessons could seem to be unpleasant, silly, or perhaps unbelievable. The Skeptic's Dictionary describes ACIM as "a slight industry" that's overly commercialized and characterizes it as "Christianity improved". Robert T. Carroll wrote the teachings aren't primary but are culled https://www.youtube.com/watch?v=RSlwOZLlUGg