I don't really know about keto, but the problem as widely understood with obesity is calories, not just calories from fat. And the easiest way to onboard calories without realising is to snack on sugary food and drinks.
Also, your point relies on the idea that the population is doing low-fat dieting. They may not be in the first place.
The only sensible diet I know of is the No S diet[0], which is so simple (and free) that it fits into the hover title of its web page.