've noticed that people in their early 30s right now are in really bad shape, even worse than people who are in their 40s despite the age difference.

I can guess why. The late 20s/early 30s is when youth fitness starts to wear off. If you were in decent shape, and mildly active, you can eat poorly and get away with it. I worked out in my 20s for strength, not to lose weight. I'd walk miles just with friends just as part of hanging out. Age, responsibility, maturity, etc changes that.

NOW, have that point in life happen during years of Covid, and you have people overeating without realizing they aren't exercising, because they barely had to think about exercise before.

I actually think that's where a lot of "long covid" comes from.