Devil's advocate suggests that a synthetic can be produced the same way every time where a cultured plant might have varying levels of the active compound in the plant. That makes it difficult to prescribe doses. As an example, suggesting a patient take 1 cap and 2 stems will be problematic for accurate dosing.
Conspirator's advocate says that bigPharma has synthesized and patented every active plant compound so that keeping the actual plants scheduled is to their benefit.
I'm fairly certain it's possible to extract psylocibin from the murshroom, giving the same advantages that the synthetic would have!
Edit0: for a more thorough look: https://www.mdpi.com/1424-8247/18/3/380