Most people paid hourly wages or monthly salaries aren't doing jobs that are entirely composed of rendering direct personal service to a customer over an extend

Most people paid hourly wages or monthly salaries aren't doing jobs that are entirely composed of rendering direct personal service to a customer over an extended period of time. Tipping is indeed customary in every service trade that I'm aware of, because it conforms to the reality that the waiter/deliveryman/taxi driver/etc. actually is working, at least in part, directly for the customer.