Consider the following story:

I was feeling a little blue. I looked at the psychiatric literature, and they were saying all this weird stuff about neurotrophic factors and cognitive-behavioral therapy. But then that night I had dinner with some friends, went to the gym for an hour, and sure enough I felt a lot better afterwards!

I would have at least three qualms with such an attitude:

First, there are different kinds of low mood. Some differences are obvious; some people are less depressed than others, or depressed for much shorter time periods. But it could also be that there are no visible differences between two people, but that for hidden reasons one person's depression will respond to some quick exercise and social activity, and another person's won't.

Second, even interventions that are known to always work can be hard to task-ify. Exercise is indeed often a very effective treatment for depression, but when you tell a depressed person "just go and exercise", they usually won't do that because they're too depressed. Having a good social support network can be helpful in depression, but depressed people can be unable to make friends because deep down they assume everyone hates them. Part of treating depression is bringing people to the point where they're able to do the simple interventions. If you get a depressed person who does have the motivation to exercise and make friends, great, but it's not a point against psychiatry that they sometimes discuss how to help people who don't.

A third problem is general anti-scientificness. Yeah, sure, you don't need to understand exactly how neurogenesis occurs in order to treat depression. But it's neat to know. And in fact exercise may treat depression by increasing neurotrophic factors, so you're not disagreeing with the scientists, just looking at it from a different angle. And for certain people it might be, in a weird way, sort of inspirational to know the science and help them figure out why they're doing what they're doing. If they want to study it, why complain?

I think most of the same issues generalize to your comment.

I would also add one more, which is that it is generally much easier to lose weight on a diet than to keep the weight off for more than a year or two. For example, of people who lost (hey, look at that!) thirty pounds on a diet, one year later they had on average gained back fifteen of them. Longer followups usually find even more of the weight regained; see for example Mann 2007. So you're declaring something simple before you've even started the hard part.