I think it's because the general public figured out they'll die in their 50s if they don't. Modern life is too easy -- a substantial portion of the population spends their white-collar lives flying a desk, "socializing" via sitting around eating and drinking, hiring people to do their landscaping and housework.. hell, you don't even need to walk to get food anymore, both restaurant meals and groceries are delivered. Sure, you might get out to a hike every weekend but that's simply not enough. For health it's pretty much a non-negotiable to go do some strenuous activity every other day, and going to the gym is the path of least resistance to getting that done.