Actually, we are service workers. The law is a service industry.
The problem is that most people tend to think of service workers as low wage workers. But doctors, lawyers, accountants, etc. are all service workers. They're just higher paid service workers. And they tend to be salaried because they are rarely tied to strict 40 hour work weeks. The nature of the work makes it very difficult for the employer to calculate the hours required on a weekly basis. A case, patient or tax filing might be particularly bad one week and next to nothing the next week. The employee gets the same money for down week as up weeks and the employer gets to avoid being hit with unexpected OT.
As for why no one takes their income in tips now? Because tips are still taxed as income and the employer still has to report them. So the employee gets no benefit from making the consumer pay a tip, just more paperwork, and there's no benefit to the employer since they still have to report how much income is being generated via tips and pay FICA on it.
More paperwork, no additional benefit.
Obviously, this changes if the income is tax free. A good modern example is carried interest in the finance world. Financiers try to get as much compensation as possible characterized as "carried interest" because it's taxed as long term capital gains which is a lower tax rate than ordinary income. To get this benefit, they defer the compensation long enough to meet the criteria. If the benefit didn't exist, they'd probably structure their compensation differently.