Because not every delivery job is a psotion where the person make less than minimum wage based on the "tipped" position. Flower delivery people are paid a normal wage. (above minimum wage)
Waitresses,bartenders, hotel maids, PIZZA delivery, maybe even couriers, are careers in which they make a tiny base hourly wage. Well under minimum wage, they rely on their tips for their income. I am lucky in that I work for Domino's, so I might make more of a base pay than a private pizza shop. I make $5.25/hour. I know someone who deliver's pizza for a locally owned pizza shop. He doesn't even make a base pay. He gets the delivery charge per customer. At Domino's, they charge the customer $1.00 dleivery charge, yet I only make $.65 customer. Domino's takes the $.35. I have not found out why yet.
So, yes, I depend on my tips from delivering pizza. So, the next time you order delivery for pizza and do not tip the driver, just remember, that is all the less money going home with him that night.