We understand how SSRIs work, but we still don't really know why. The medication takes effect immediately, in terms of what it does to the amount of neurotransmitter hanging out in the synaptic cleft, but it often takes two weeks or more for patients to experience relief. So relieving depression is not simply a matter of "boosting serotonin" -- it's something we don't fully understand.
There are hypotheses, some better than others, including the idea that SSRIs help boost neurogenesis.
But I think the answer probably has more to do with the supernatural entities in the brain, of which neurotransmitters are only the physical manifestation.