I agree with you, and I should note that not all of the drivers do this. The manager will also always give you a free pizza on your next order if your complaint matches up to the delivery times (which are all tracked via computer).
I've seen this happen at two different stores from the same chain. It usually only happens with the drivers whose entire livelihood depends on how much they make. These are the drivers who are making a career out of delivering pizza, and for some reason they take it as a personal attack when they get no tip after delivering a pizza in a timely manner (30 - 45 minutes).
The main point I was trying to make was that tipping well is an incentive for the driver to keep you happy. The driver is trying to maximize his profits, so he who tips the most gets his pizza first.
Also, not every city has multiple pizza chains that deliver. Where I went to school, there were two chains in town, both of which delivered. The other chain, however, doesn't usually deliver (they only do so in college towns), so their delivery service is terrible without trying to be.