Science

This has been known for more than a decade (this study is just a review of existing research), and is not directly related to the efficacy of SSRIs (although the serotonin "theory" might have led to their adoption, idk).

For a while now, as I understand it, the view has been that SSRIs cause the downregulation (decreased new production of) serotonin receptors in reaction to the excess serotonin buildup they cause in the synapses (this is why they take a few weeks to kick in, despite increasing serotonin levels almost immediately, and is also why they will desensitive you to psychedelics like LSD, which act primarily on serotonin receptors). They decrease overall serotonin signalling, which in turn has downstream effects that alter certain activites of certain regions of the brain that are implicated in depression. (This is based mostly on discussions I had with my psychiatrist a few months ago, so if there are any actual experts here, correct me if I'm wrong.)

That being said, SSRIs suck ass, their side effects are awful, and I think it's medical malpractice to make them a first-line treatment tbh. It's really mind-boggling that they're the go-to given how much shit they fuck up.