mainstream media doesn't = the general population actually participating in the popular things. it only means they WATCH the popular things. you said it yourself, the average person is fatter and lazier than before. how are they fatter and lazier than before if they're all doing fitness/lifting and martial arts? it's because they're not.
It doen't make sense to you because you're drawing a surface-level conclusion.
Going to the gym and having a poor diet aren't mutually exclusive; there is generally speaking some correlation between the two, but exercising and eating are two different behaviors. Plenty of people who work out still have excess belly fat. The phrase 'you can't out-lift a bad diet' exists for a reason. Hell, plenty of athletes whose livelihoods depend on eating healthy food can't even maintain both, probably one of the reasons they balloon up when they're retired, because they don't have that strict training regime to at least partially offset bad calorie surplus anymore.
You can look at it like this: there are different moving meters that create homeostasis: diet, exercise, and everything outside of exercise (work, sleep, day-to-day life), which is the vast majority of your life, even considering if you'd exercise 2 hours a day, 7 days a week. 14-15 hours of exercise would put you firmly in the not-quite-professional, but definitely competitive athlete category, and it would still amount to less than 10% of your week. We also sleep less, partially due to the changes in our environment. Lack of sleep increases ghrelin, the hunger hormone and also makes our body shift more to fat-storing mode.
So while exercise is important, it's the least influential of those 3 elements. It's not a monolith, so I'm sure it differs per country, but it's perfectly possible that fitness participation has increased in some places, while the
average person continues to gain weight because environment and diet outweigh what those few hours of focused exercise can compensate for.
Kinda made me think about the link between masculinity and exercise, because Eastern European countries are generally viewed as more traditional and masculine, but they are less fit per capita compared to Northern and Western Europeans.