[po]

Because it's exactly the same thing.

A bonus is paid to you by your company (evaluated by your boss) and is more like profit sharing.

A tip is paid directly by the customer. The customer has no way of knowing if you are being paid a living wage or not. They have different incentives if they are a regular or not. The waiter at a busy restaurant will see hundreds of customers a day, but the customer may only interact with this waiter once. The waiter understands the house systems and knows how to maximize tipping. These are all ways that the waiter has an information advantage and can wield it against the customer. Maximizing tipping and providing good service to everyone are seldom the same thing.

People like to think that tipping puts the customer in power but it really just sets up the customer to be gamed. This aspect doesn't exist when we're talking about bonuses with your employer.