While the rise of technology has lead to less activity in children, and adults too I think this answer is much bigger than "kids these days don't go out and play". This thread is the answer, food has changed. And not for the better. Food isn't healthier now, sure there might be more healthier alternatives, but it's not accessible to anyone. The "food police" say you need to offer apples as an option for a kids meal at McDonalds, but that doesn't mean people are ordering it and more importantly you're getting McDonalds, it might be healthier but it's not healthy. Not allowing super-sized meals and drinks isn't healthy it's healthier.
Also look at the majority of the foods listed in this thread, fast food, junk food, processed food. You mentioned how soda with high fructose corn syrup tastes bad, that shit is in everything that's packaged.
A few summers ago I worked at a "resort" in Alaska, there they do a lot of outside hiring, throughout the US and some European countries. About 2-3 into working there almost all of the Europeans had gained noticable weight and their skin was breaking out. A small anecdote but a lot of other 1st world countries have tighter restrictions on food additives.