Even if you do decide to work out, it tends to be the first thing you drop when you don't have time. Or you worked late. Or you forget your gym clothes. Or you're just too exhausted to go.
We used to be active, our jobs required motion, we had to walk or cycle to work, or even take public transportation - which requires you to at least walk to the stop and back.
Now we literally sit on our asses, day in, day out, and wonder why so many people are obese.