Loads of good explanations here - not much original to add, but just to reinforce:
Netflix, social media, computer games - many children and adults spend most of their free time doing these things rather than more physical activities.
Fast food or takeaways: I have about one takeaway a year (mainly they're so expensive - I'm no angel!!) but I learn by reading MN that some people have them more than once a week and some almost every night. I count Gregg's and similar into this category.
Fizzy, sugary drinks - say no more. Luckily for me I was never allowed them and have never got the taste for them, and I'm grateful for this.
The idea that you can seriously eat: full breakfast (fry-up and/or toast/croissants/cereal/pastries) then lunch (full meal or carby stodge of some variety plus pudding) and supper (big meal or takeaway, large portions and pudding). Plus snacks, of course. NO YOU CAN'T! This is propaganda. There is just no way it can be made to work with the recommended calorie intake for men and woman. Something has to snap (probably one's knicker elastic).
Lack of willingness to take any personal responsibility - you can see it on this thread from the people who avoid the OP's point and accuse pps of'fat shaming'.
Plus, in some demographics, as pps say, it's become so normalised. People think waddling along out of breath when you're young is normal - it's not! So there's no peer pressure or aspiration to keep fit and healthy.
I don't think we can let people off by saying they don't know about healthy eating. They must have been living on the moon - they just don't want to hear that message.
The wartime generations had to make do with rations and, horrible though it was, they were healthier for it.
I think the latest thinking is that fat (not trans fats etc) are good in moderation and we shouldn't demonise them. Protein and fresh plants - great. The real baddie, I think, is carbs. They are responsible for so much damage in terms of diabetes and raising cholesterol. Shame they're my favourite food group - but at least I know that and I care enough about myself to limit them.