It's the thorn on the free society rose. Having freedom of choice and lifestyle often means people choosing things that are easy and convenient. In expanding Newton's law of equal and opposite reaction to a broader spectrum, things that are easy and convenient can often have disastrous consequences. People are fully aware of the consequences. It's likely true to say that most every fat person is fully aware that their obesity can cause a plethora of health issues. However, there's a evolutionary design flaw in humans (and most mammals) of "it won't happen to me." There's also the false belief of "I can stop whenever I want." We tell ourselves this as a survival or coping mechanism. This can be so strong that people will fight to the death to protect it. Thus, how do you solve the issue without significant, deeply infiltrating restrictions on people's every day lives? You'd be looking at a 1984 reference of food rationing to make that happen and we'd be ready to go to war long before it came to that point. Calling them lazy won't work because they're mentally far beyond that point.
It's not like smoking where you can stop smoking cold turkey and survive. You can't stop eating or you'll die. Going back to easy and convenient, we're living in a much busier society (in terms of family dynamic) than much of history. If you go back 50 years, only ~40 percent of women were in the workforce. Today, that number is near double that. Not a patriarchal rant here at all, it's just a reflection of how our economy has changed to where a single income household is no longer doable for most families. When you had a parent/spouse home all the time, it was easier to have a consistent meal plan. We didn't eat out nearly as much then. We ate healthier foods from home. When both parents are working, neither of them want to come home and stand in front of a stove for 1-2 hours and deal with the cleanup afterwards. This is compounded by the time problem such as getting off work at 4-5 pm, little Timmy has a soccer game at 6, and little Tiffany has a ballet recital at 730. When are you supposed fit a dinner in there? You don't. You grab fast food because it solves both the time and stress/tired problem.
On top of that, we've transitioned from a manufacturing based workforce to a retail/service based one. This means less physically demanding jobs, more jobs sitting down behind a computer/desk/register, and so we're not getting the same physical exercise in our daily lives as we used to. You don't have to walk in a place to get food with a drive-thru. You don't even have to walk in the grocery store to get groceries with things like clicklist / curbside pickup. It's things like this we don't often think about when it comes to this problem.