Medicine just isn't that far advanced yet. Imagine how much work it took just to get to this point, where we even know that neurotransmitters exist! I think there is a lot of hope for the future, though...we are learning so much about the gut these days.
It's probably not just the SSRI causing your diarrhea. There is usually a combination of factors for most afflictions and while it could be exacerbating things, it is unlikely to be the main cause. Does taking Immodium help?