Car dependency. The shift over the last 60-70 years to push everyone to drive more and more and walk, bike, etc less has had an absolutely massive impact on obesity and being unfit.
Compare the obesity rates in any area in the US versus types of transport and you'll see a huge correlation when other factors are fixed. e.g. https://pubmed.ncbi.nlm.nih.gov/28322880/
I was walking to school with my older sister at age four. At age 6 I was doing a fairly long bus ride with her (and we walked down to the bus stop of course). The rest of my school life was mostly riding and walking to school. And this was completely unremarkable. What was remarkable were the kids who were dropped off at school; I viewed it as a somewhat eccentric practice.
I remember begging my parents to let me bike to school because all the "cook kids" rode their BMX bikes to school and the bike racks were overflowing with kids who were biking to school. I remember being made fun of in the summer for taking the bus instead of riding my bike.
I don't remember seeing bike racks at any of the schools my sisters kids attended.
Oh, and one big thing I don't thing anyone mentioned yet: Smoking. When I was a kid, a lot more people smoked. That has a well known side-effect of reducing calorie intake.
Unfortunately management and company culture tends to frown on doing these things.
- The tamping down of phy ed classes and general removal of exercise from elementary schools with more of an emphasis on learning. If you are not ok shape in middle school, it leads to learning issues. One problem begets the other
- Shifts in parenting. When I grew up in the 1980's, our family ALWAYS ate dinner at home, together as a family. My mom got home early from work when me and my sister got home from school and started dinner before my Dad got him from his job. We RARELY ate out and it was looked as a privilege, not something that happened often.
- The shift from the nuclear family. Most of my friends are married and both parents work full time, their kids are heavily involved in sports, and after school activities. Less time for family, less time for organized meals, less time to prepare good food for your kids. The basic idea now is "just get something in them and get them to soccer, hockey, gymnastics practice, etc."
Likewise, the parents have little or time to properly feed themselves either. Same thing, grab something fast, shovel the food into your kid, then get something fast on the way home before you have to jump back on your laptop and finish the last bit of work you didn't get done because you had to get little Bobby to practice.
This is a situation that never gets any better. Your kids have more activities, you constantly have more work to do. The go go go go attitude of our culture has left the nuclear family and our nutrition in the dust.
Humans are lazy by nature. Make it easier to eat unhealthy foods and we'll do it. Its all we've done for the past 40 years.