Isn't it more that we don't understand why it works (i.e. what kind of connection serotonin has with these mental illnesses) rather than the mechanism of action itself, which is believed to be serotonin staying longer in the synaptic cleft by way of inhibiting reuptake, leading to repeated stimulation of the post-synaptic receptors?

I was under the impression that SSRI's were specifically designed to target the serotonin system.