The synthesized substance is not exactly the same as that which is naturally obtained. Just like how store bought 'vitamins' are inferior to those that come directly from fruits and vegetables, or, as someone mentioned earlier, THC that comes from a lab is different than when it is sourced directly from the plant.
What can be created in a lab is similar, but does not contain every single chemical that is produced when the source is a terrified person. The comparison to vitamins is valid because they also lack various naturally occurring substances in very small amounts that are extremely important.