Yes, because a lot of times the drivers don't get all of that delivery fee. For example, my husband is the General Manager of Qdoba. They also cater orders and if requested, will deliver them. There is a $25 delivery charge, which includes delivery and set-up. However, the person who delivers the order only receives $15 dollars of that delivery charge. I would say at least 60% of the people he has delivered to does tip extra. However, there are some that don't. He had a $1500 catering order that he delivered, and they didn't tip any extra at all. He's also driving 20ish minutes away and they haven't tipped either (happened today).
Now, I realize that is a lot higher delivery charge than say your $2-$4 dollar pizza/Jimmy John's/etc delivery, but I am guessing those delivery drivers don't get the entire delivery fee either.