The restaurant I manage pays drivers minimum wage (by which I mean minimum for non-tipped employees) plus tips. Drivers make out pretty well when it's busy.
There is a charge for delivery, part of which goes to compensate the driver for gas, wear and tear on vehicle, etc, the rest to the company to cover the monstrous insurance costs involved in employing drivers.
Tips are encouraged but not required. We tell customers we do not consider the charge to be a tip, but some choose to see it that way anyway. I have some really fun conversations with jerkwads who are trying to justify not tipping. To be clear, they're not jerkwads for not tipping, they're jerkwads for trying to pry into our payroll information or weasel out of tipping with some flimsy justification. No, sir, we will not reveal how much our drivers get paid per hour, or per delivery. We do not care why you think you are entitled to this information. You've already decided to stiff, just do it and get on with your life.
Personally I tip based on service. A good tip can be as much as 25%. Depends on speed, courtesy, accuracy. I will stiff if service is bad enough.
I'm not interested in philosophical discussions about why jobs are considered tip-worthy or not. There's no real reason, it's just the culture we're raised in. Why is nudity a taboo? Same reason.