I'd say a big portion of the problem is our car culture. People sit in cars for like an hour in rush hour to go to work, sit at work all day, sit in cars to get home, and then are tired after a long day and sit on the couch watching TV.
Despite the fact that I spent a decent amount of time just sitting around as a college student, I still had a part-time job that required me to be on my feet and had to walk to class every day.
My weight was more or less fine (averaged in the 130s/lower 140s), but started to become an issue (creeping up into 150s, 100% attributable to weight gain in the abdomen, waist, and thighs) literally immediately after accepting a 40-hour-week office gig with a 40m-1hr commute. (Which translates to about 10-11 hours of my day being unusable for physical activity, and leaves me even more tired at the end of the day - I have metabolic/mitochondrial issues already.) And my GI system has started going to shit because of it - I need to get a colonoscopy at 27.
Now that I have full healthcare benefits, I actually have a need to use them.
Looking at my family history, it's clear I need to really change my diet and carve out some time for exercise, but damn is that hard when the part of my day dedicated to work pretty much takes all my energy out of me.