Why do people tip waiters and waitresses 15+ percent but won’t tip a driver the same?

We use our own personal vehicles to deliver $200 worth of groceries or more regularly, but people have a problem with even tipping 10% of what they paid at Walmart. That has always bothered me about these jobs. What do you think? We do twice the work of what a waiter does at a restaurant, but people have a problem tipping the same percentage.