I have a little insight on this. One of my best GF's son worked in a grocery store. They would rather take your bags out to the car and get outside than be in the store customer after customer bagging groceries. Also? The main chains in the US do not ALLOW you to tip them. If they accept they can be fired. It is a courtesy.
So next time, consider you are giving them a little break from the monotony if you let them walk you to the car