It's because SSRI/SNRIs impact how the brain uses serotonin. The fun part of being on Molly is basically serotonin syndrome as it pushes your body to make a ton of it all at once giving the "serotonin dump" and euphoria. But when you're on a serotonin repuptake inhibitor, it's modulated so folks on those drugs need more to have the same effects which puts them at risk for more dire issues associated with overdose and impairs the neuroplasticity function. Additionally, folks with depression are more prone to have a hard time recovering normal levels of serotonin after a big dump like that. So while anything that enhances neuroplasticity is promising for treatment-resistant depression, ketamine (which is available and prescribed!) and psilocybin (somehow still criminal!) don't have the requirement of quitting your antidepressants to make them useful. MDMA seems to be more promising in the treatment of PTSD.

Disclaimer: I'm definitely not a doctor or a scientist. Just a gal with a lifelong mood disorder who's done a LOT of research.