Additives or preservatives aren't likely to be the core issue. At the end of the day, by far the most significant factor for weight change is calorie intake vs expenditure. The new drugs like Semaglutide fundamentally work by significantly suppressing hunger and increasing satiety, for instance. I'm not sure what the precise data looks like, but in general, people are less social, go out less, spend a lot more time sitting on their phones or computers, and are generally a bit more stressed. All of the is going to contribute to eating more and moving less.

There's also a big link between lack of sleep and both obesity and cancer (and dozens of other health problems).

Western society has glorified not getting enough sleep as though it makes you manlier or better in some way. It doesn't. It just makes you die sooner.

We've moved to an "always on" society with the proliferation of the internet. With this comes disrupted circadian rhythms and even more reasons/excuses to not follow a regular or beneficial sleep schedule.

Sugar and refined carbohydrates are two big culprits. Then changing the balance of HDL-cholesterol to LDL-cholesterol changes with weight gain making it all worse and possibly leading to a positive feedback loop. The historic denigration of all fats, good and bad, helped to further tilt the HDL to LDL in the population making lots of people less healthy. It isn't HFCS, it is the over use of sugar in most of our food. This is especially true in North America, but then we exported much of the same food tech to the rest of the world who did the same.

All that processed food? Full of refined carbs and sugar. Drinks? Often full of sugar. Cheap food? Usually highly processed and refined, so more sugar and refined carbs. You need carbs for energy and fats to keep everything going with balance of nutrients and protein. Any of that out of balance and health suffers. Too little fat can even kill you (rabbit starvation/protein poisoning). The modern diet in North America is terrible because we were told good things were bad and carbs were good. So we ate too little of the good fats, too much of the refined carbs, and too much sugar. Now were here, increased heat disease, diabetes, obesity, and cancer.

Don't forget sugar. I've recently visited the US and their bread (if you can call it that) could easily be classified as cake where I'm from.