Eating healthy is a lot like quitting smoking. You are addicted to fat, sugar, etc and when you stop your body screams for it. Willpower is not something our society values so most people simply don't have it.
Top that off with the fact that losing weight means being every so slightly hungry. That's another willpower issue.
It's a long road.
Plus, people have stayed fat tend not to burn as many calories with a given amount of exercise (one of a few reasons why the simple calories in v. out doesn't work for many--controlling calories out is a problem for people not used to being active). In my own case, exercise simply does not cause me lose weight. I will tone muscle with very little fat loss.
Know what works really well, though, however slowly? High-fiber food, especially w/ whole grains. Not fiber pills, and not Cheerios, but stuff like oatmeal, tabouleh, beans, etc., that keeps you full, has plenty of minor nutrients, and leaves you not prone to sugar/salt cravings through the day. But, see, that shit requires planning, time, and effort.
Who has that? You can half-ass it with prepared foods, but even that requires planning, effort, and not eating something else instead. Is that unamerican, or what?
If a pill or injection can work, great. But, that's not going to keep you from yo-yoing afterwards, so it's really not great, unless you're basically bedridden. Unless, of course, you are the seller of said pills, injections, or whatever.
Then it's always great. If fad or Rx A doesn't work, there's fad or Rx B to try. With enough options, and enough people wanting a quick fix, there will be consistently many customers. Being a prescription just means it might take a bit of doctor shopping.
But you could eat McDonalds garbage every day and so long as you ran a caloric deficit you would lose weight.
In theory, you could. The reality for many, if not most, is they'd get more sick, and not lose any weight, because their body would be conserving energy, and they wouldn't have the stamina to be active enough use up more of them.
And, ironically, McDonalds has among the best food selections of any large fast food chain, despite being considered the canonical evil.
Most people that aren't obese due to depression would be fine if eating better, rather than at all worrying about eating less. But, surely, you've seen when it's been tried to give kids in schools healthier meals, and it fails, right? Tons of adults are the same way. They're so used to the crap, that they prefer to keep eating it, and stay how they are. Mountains of national medical debt, here we come!