We are going to ignore the fact that this is not healthy for you, but let's look at why it is not healthy for the planet.
So, why is it bad for the planet? Think about it, first you drive to get it (or someone drives to bring you it) and put pollutants into the air. Then they package it, and repackage it. A hamburger typically comes wrapped in paper. Then all your food is put in a bag. Now, because of what it is made out of, and that it has food crap on it, you can't even recycle it. In the garbage it goes. Then there are all those small packages of ketcup and other things that hold tiny amounts of goo in plastic that gets thrown away.
So, not only is fast food bad for you, it is bad for the earth. Yet, I still think I will be getting my McD's for lunch when I work long hours.