Your statement is accurate and reasonable for servers, who are employees of the restaurant, and are guaranteed to earn at least minimum wage.
But we are talking about delivery drivers. Drivers are generally contractors, not employees. There is no minimum wage for contractors. Further, contractors are responsible for their own expenses. The IRS says a mile of travel costs $0.655. DD typically pays a base rate of $2 per delivery, whether around the block, or 20 miles away. That $2 fee covers 3 miles of expenses, which is about a 2-mile delivery, plus travel to the store.
Typically, the driver ends up paying all of the base pay in travel expenses. The only part of his compensation he actually gets to keep is the tip.