The evidence for antidepressants is not based on the serotonin theory of depression. It's based on blinded placebo studies. The serotonin theory was posited after MAOI antidepressants were discovered. It's been argued that antidepressants aren't significantly better than placebo. There's a good discussion of that here: https://lorienpsych.com/2020/10/25/ssris/#3_How_effective_ar...

There's also a background debate here over whether depression should be seen as biological/chemical or environmental. This comes up in the article toward the end:

> Although viewing depression as a biological disorder may seem like it would reduce stigma, in fact, research has shown the opposite, and also that people who believe their own depression is due to a chemical imbalance are more pessimistic about their chances of recovery.

The thing is, if you're not a dualist, there's no reason depression can't be chemical and environmental. Clearly life circumstances can lead to depression and making changes (diet, exercise, leaving a job, leaving a relationship, undertaking therapy) can be a way out. Most doctors understand this and that doesn't discredit medication as a potential option.