SSRIs specifically have been shown to be less effective than placebo in nearly every study and NO ONE really understands how/why they work.

What? We have a very clear understanding of how/why they work. It's in the name, selective serotonin re-uptake inhibitor. We know that people who suffer from depression have lower levels of serotonin. The hypothesis being that increasing those levels should reduce the depression. The drug works by selectively blocking the re-uptake of serotonin in the brain, causing more of it build up. That's how/why it works. Research does support that hypothesis. You might argue we don't know the full extent of the side-effects though?

I'd also be curious to see some of those studies because I have never heard that before. I am aware of some studies that show placebos are pretty effective in the early stages (6 weeks in) when compared with another alternative treatment (st john's wort). However, that difference grew smaller later on (8 weeks into treatment). It would be worth seeing if that is a trend that persists because we need the medicine to keep working.