are drawn by custom and tradition, not necessarily by logic.
Customs and traditions are not really logical things, but we are expected to do them anyway. And we look bad not doing them.
But also consider this - employers and employees take those customs and traditions into consideration when making wage decisions. The pizza place knows that its drivers are going to get tips, and so it will offer a correspondingly lower wage - and the driver accepts the lower wage based on the same expectation. If the employer was paying it, he'd pass that cost on to you in the form of higher prices.