The amount of people that meet the requirements to be a restaurant worker are vast. If they want to replace a server, host, or busboy what are the requirements needed? Be 15 or 16 years of age? You can take someone that's never done job, has no experience or education in that field, and train them in a few hours.
Now do that with a doctor, nurse, HVAC tech, or accountant. What are the requirements to get a job in those areas? Education and certification. A skilled job.
I don't think anyone is saying that a server, the job not the person, isn't important to a restaurant. Just that the job is easier to fill because there is no required education or certification required for them to be hired to do the job.