I don’t know why you’ve been downvoted. The amount of knowledge we know of the brain than is significantly much lesser than what we don’t know. Heck, we’re still not sure if SSRIs work by increasing serotonin, reducing inflammation or increasing plasticity. Maybe all 3 are related.

Evolution has had millions of years to work on its craft. We’ve been studying it for an infinitesimal fraction of that time. There’s nothing surprising here.