Um, they kinda do. In my experience it's generally not "advice" in the same way a doctor gives it, but it's been more along the lines of, "I took X. Y happened." Not, "You should (not) take X because it (worked/didn't work) for me".

People keep saying we should _normalize_ talking about mental health, and what you're suggesting goes against that. If we were discussing heart health and someone were to talk about their exercise, diet and medication online, would you say, "Hey, wait a minute! You're not a doctor. You can't say that." Of course not. What you're advocating for is an overly paternalistic approach to mental health issues, as if people are too dumb to listen to different viewpoints and come to their own conclusions.