Being sedentary is not a good thing in high doses.
The modern man is enabled by an abundance of tools and resources to sustain ourselves while remaining wholly inactive than any of our ancestors. Chronic inactivity is a byproduct of refrigerators full of food, house walls keeping away pests and predators, desk jobs, and take-out. It is also the cause of (or, at least, a major contributor to) many of the diseases that plague our modern post-industrialized society. Lack of exercise (or even lower forms of ‘simple activity’, like folding clothes) is a contributing factor to Chronic Low-Level Inflammation.