We sort of do know some of the mechanisms why they work: they increase BDNF and stimulate certain serotonin receptors, allowing new and healthier neural connect

We sort of do know some of the mechanisms why they work: they increase BDNF and stimulate certain serotonin receptors, allowing new and healthier neural connections and associations to be formed, helping people more easily snap out of old ways of thinking, similar to how psilocybin and other newer "drugs" help people snap out of depression. That's why antidepressants are ideally combined with CBT or other therapies that help people change their mindset.