Sale tax is calculated for you automatically, but tipping is considered a customer's responsibility. Why do you believe this is so?

This is because the business is required to pay sales tax to the government regardless of whether or not you pay it. When you pay sales tax, you are not actually paying your taxes. You are paying the establishments taxes which they will be charged because of you. There is no consequence to the business if you do not tip, and although it is unethical not to do so, it isn't required. It also allows the consumer to feel like they have power over their service because if it is sub-par, their waiter/waitress will receive a lower tip.