Anyone who has worked "unskilled" positions can tell you that every job has a learning curve and experience counts for a lot.
This is particularly true in jobs that require a degree of physical endurance and manual dexterity. Picking a vegetable is easy. Picking a thousand vegetables an hour (without bruising the produce or ruining the plant) for eight hours a day is quite difficult. And skilled workers are far more lucrative to the farm owner than clumsy neophytes.
What often defines a service worker as "unskilled" isn't the work, but the degree to which automated capital and real estate ownership are integrated into the workflow. The more leverage the employer can exert over the hiring market, the more easily they classify labor as "unskilled"' and downgrade the pay.