Let me ask you this...
Why should anyone have to bribe a person to do their job correctly?
If other professions decided that this form of extortion would net them higher salaries, would that be alright with you?
After taking off on a flight from New York to Los Angeles, the pilots pass a hat. If the tips are sufficient, they take the plane to L.A., if not, you end up in Cleveland.
Because 75% of their job is dealing with people like you?
You pay many different types of businesses commission for the service they provide- car salesmen, real estate agents, electronics salesmen. They all earn commission for servicing you by helping you find the product you want. The only difference is that you are in control of the rate of commission instead of letting someone else tell you the commission rate built into the price.
Because they are not being paid decently in anticipation that they will be getting tips.