Do you suggest that is bad?
First, have you ever seen cheetahs? Even in their natural environment, they laze about almost all day.
Second, we are intrinsically lazy, and not because "the modern world has corrupted us".
Notice I am not claiming that our bodies were in fact designed to sit forever, but can we really declare it is "obvious" that sitting for a long time is against our nature?
From an evolutionary standpoint, it makes sense that we would preserve energy. Food wasn't always plentiful, and unnecessary calorie burning would work against survival and propagation of the species.