The side effects of SSRIs cause people to stop taking them, so if you have a medication that can actually alleviate the symptoms of depression but causes horrible diarrhea, you're swapping one bad thing for another and potentially causing untold bowel problems. SSRIs start working immediately but people may not notice the difference for several weeks. I'm not sure if the SSRIs can be as effectively absorbed if they cause diarrhea though. I took sertraline starting the dose several times but needed to stop as the side effects were so bad. I know that ending SSRI treatment has side effects too that stop people stopping, which is bad too but given the fact they are proven to help many people, it's good they can be prescribed.
An ideal situation for a pharma company is the person can easily get started on the medication and only gets side effects if they stop taking it + visible benefits of taking it. If pharmas can develop drugs with only the withdrawal side effects then it#s huge money for them, even more than presently.