if some unknown element was making everyone's internal thermostat aim for more food it would explain a lot.
Science and capitalism have created incredibly delicious foods that are nutritionally lacking, hyper optimized for (against?) our now mis-aligned reward system. In the west, calories are not scarce and the most delcious foods are far from the most nutritious. It will take a long time for our genes to catchup.
Mass producing delicious, cheap, but low nutrition food is profitable. Companies have gotten very good at it. That's the real big change.
we got the abundant food and the largely car bound live cycles and it still kept getting worse for decades after that point. I suppose it could be generations growing up only knowing this and so habituated to it more?
The "public presence" of society has diminished due to the internet. You no longer need to put effort into constantly looking your best because social media helps curate your appearance. Going to Walmart is now so relaxed that you can wear pajamas. Putting on your "best appearance" occurs elsewhere in curated ways (i.e. facebook/instagram posts and careful selfies). You can "partition" your social life so that the people shopping at walmart see pajama-you while the Tinder matches see someone totally different.