For one, it's not exactly a popular movement. A lot of attempts to make progress on it get attacked from the right as being "nanny state" on from the left as shaming fat people. There is a small but extremely vocal contingent of people who even deny the science regarding health and obesity.

Two, the issue is complicated because weight loss is often hard. Not complicated, exactly, but hard. And access to healthier foods, having unhealthy foods shoved in your face all the time, access to regular checkups, (heck, even the way that a particular degree of obesity affects a person, both socially and biologically), are tied up in class and race. The most efficient place it intervene is probably with children, before they gain weight. But this again gets attacked from both the right and the left. And more than that, most parents of overweight and obese kids think their kids are normal weight.

Three, the foundations of the epidemic are to an extent written into our food system and way of life. We subsidize corn syrup and animal feed. There's been a shift away from manual labor and toward sedentary jobs. Large swaths of the country are "unwalkable" either because of the distances involved or for safety reasons. Heck, I live in a big city and it's not uncommon to find yourself in a part of town with no sidewalks, or where you have to walk an extra few blocks to find a crosswalk.

I don't want to get into moral judgement or what people "should" do, but the fact is that a large segment of the population will do what's cheap and easy. If soda is cheap enough to justify how good it tastes, if processed meat is cheap enough to justify how easy it is, if walking somewhere is a pain in the ass because there's no sidewalk but gas is cheap, we're designing a certain lifestyle for a lot of people.