What's wrong with being physically fit?

Skinny people aren't always in shape or active for that matter. And physical activity reduces your risk of a host of chronic health conditions and improves sleep and focus.
I also think people are confused. Gaining weight isn't a problem. It's the excess weight from consuming highly processed and calorie dense foods coupled with physical inactivity that are. Historically, we loved fat and sugar since they were in short supply. For this reason, excess weight was admired as it was seen as a sign of wealth because food scarcity could strike at any time. Anyone with a consistent enough food source was envied.
We don't have the same problem in the west, today. As we can easily cultivate nutrient poor but calorically dense foods making weight gain more effortless. People are also significantly less active (which is a major killer).
However, most centenarians are a thin/normal weight and they are consistently active. They might not be gym buffs but they are more regular with their movement by doing chores and walking miles daily or gardening and they have a more plant-based diet. Might I add, weight training certainly helps seniors maintain their independence over time.