As someone that has done both jobs, driving was A LOT more work. I think they should be tipped more, honestly.
First reason: well, let's say for a second that they did the same amount of work (they don't). People are so brainwashed to think that waiters struggle that almost EVERYONE tips them, whereas drivers rarely ever get tipped no matter what (and it's usually a few dollars at most when they do).
I think not tipping drivers comes from a few false assumptions:
1. The delivery/gas fee, if there is one, goes to the drivers. Not always. Sometimes it does, sometimes it doesn't. Still doesn't cover wear and tear.
2. Drivers are paid minimum wage. FALSE! Again, some are, some aren't. Some drivers get paid just a little more than waiters per hour.
3. Waiters serve, get finished plates out of the way, get drinks, have to fix incorrect orders, etc., while drivers only drive. Once again, WRONG! Most drivers also take orders, clean, make the pizzas, deliver them, have to drive back to your house to fix orders and miss more deliveries, etc.
Waiters get to sit inside nice, warm buildings, while drivers have to weather rain, ice, sleet, traffic, muggers, etc. Waiters also know where people are seated. Drivers sometimes have to go door-to-door to find the right house, as many aren't numbered, or the numbers are out of order (I once saw 2026 next to 87, and 96 followed by 79 next to 87). And when they do find the right house? People fall asleep or take showers after they order all the time.
If the food is bad, people usually don't blame it on the waiters, they blame it on the cooks. If you have a delivery run that took an hour, and come back to find another six deliveries waiting for you? You'll get blamed and stiffed when one or two of them end up getting cold because you can only drive to both ends of your range so fast.
There are so many other reasons why, but having done both, I'd say that drivers seem to do more work, but get appreciated less.
Also, if drivers get into an accident, pretty sure insurance won't cover it.