The point is that SSRIs (and reuptake inhibitors in general) are extremely crude. It's like using a dull rock instead of a scalpel, or full body radiation to remove a mole on your hand. They have a lot of secondary and tertiary effects we don't understand, nor do we truly understand the primary mechanism.

We don't know exactly what these drugs do to the body, how they do it, or what the consequences really are.

They have good outcomes for a small section of the population and that's about all we know.

One could argue that over-prescription of such a poorly understood class of drug is far more harmful.