If we get really, really good at fine-tuning our shamanic alchemy and we can play the brain exactly as we want; cool, but... there is something that the mushrooms tell you that a synthetic, perfectly modulated chemical won't.
The mushrooms are going tell you some stuff you may not want to really hear when you're high. Like, hey you're fucking up the planet here. Hey you're cheating on your girlfriend there. Why haven't you talked to me in the last three years? ... The sacred plants have agendas.