Yes, usually the delivery fee goes straight back to the company. In other cases, it or a percentage of it is paid to the driver, for the very reason that many people do not tip well so this is their assurance that the driver makes a decent wage. I personally know a pizza shop owner that does this for this reason. People regularly under-tip (or don't tip) pizza delivery drivers, it's a fact. It's also an added service to bring food to you, and should be compensated as such. Even as an owner of a catering company, I get tipped for delivering the food, that's how the industry works (If I have employees working with me, the tip is split among us). But I don't get 20% unless I go over and above what they asked, because we figure in the cost of delivery on the bill.
He most likely loaded the pizzas himself, drove carefully to get them to you still hot and in tact, and then will be unloading them himself, too. He also did a certain amount of quality control to assure you get the pizza you ordered. There is a certain amount of skill involved in delivering that much food at once, but those of us who do it for a living do seem to make it look relatively effortless, don't we?
I'm glad you asked this question, many people wouldn't care at all. For 25 pizzas, let's assume about $10 per pie after they discounted their product and delivered it to you, I would tip AT LEAST $30-40.