There are things that are causal and have little to no effect and things that are causal and have a large effect. I think determining whether and how much something matters, especially when the target audience is "all humans" is a worthwhile endeavor. So it may seem obvious to you, but many many important scientific findings were because somebody looked a little deeper at something that seemed obvious to everyone else.

Because for it to be effective it must have reasonable cost compared to the benefit. The cost for such a sweeping lifestyle adjustment seems quite high, and the authors have not showed any benefit to interventions.