For one, serotonin receptors are _everywhere_, notably the gut, which also has a strong relationship with emotional state. And as you point out, they don't just affect serotonin. Given we have a whole bunch of drugs that attempt to be serotonin selective, but where each one affects different people in different ways, it seems likely to me that the non-serotonin effects are likely significant.
Plus, what's the reason so many of us are anxious, depressed, neurotic, autistic, ADHD, etc. without obvious cause? Why would a huge chunk of the population have a imbalance of brain chemicals?
Ultimately I suspect my GAD/OCD is probably environmental, but environmental in a way that I can't easily identify or fix. Maybe it's the unnatural desk job oriented lifestyle I have, or maybe it's plastics in my blood, processed foods, lack of religion, social media or something else. Or maybe the neurotransmitters just sometimes need a kick up the arse to get them back into proper functioning.
All I really know is sertraline helps me live a normal life, and for that I'm thankful.