The biggest problem is food poverty and lack of cooking skills.
While it's not the entire reason, it's a big factor. I know lots of people who like cooking, including myself. I tend to reach for the salad drawer for inspiration when deciding what's for dinner. My DH who hates cooking, roots around in the freezer for a box of something to throw into the oven.
Add into that when you are struggling financially, the beige frozen stuff is cheaper, and if you've fussy kids who won't eat vegetables then that's just money you don't have, wasted. In turn, those kids grow up used to those foods and cooking practices and are doomed to repeat them.
Takeaway portions are massive. Too big for one person and yet most people would eat that in one sitting because those portions are what they are used to, and anything less will make them hungry.
Exercising can be difficult to fit in, but there are ways to try. I'm gone out of the house 12 hrs a day and for months of the year I only see daylight at lunchtimes. I try to do a run or walk at lunch time.
My cousin will tell you that she gets no opportunity to exercise so that's why she's massive (her words) and that's why she needs to go to Turkey to get dodgy gastric band surgery, but she lives with her adult kids who both go to the gym almost every day, have taught themselves how to cook nutritiously and healthily and have tried to get their DM to do some. They used to be obese too until they chose a different path. She just doesn't want to.