Public school teachers (in the US) are required to have college degrees and get a certification. And if too many are out sick, they call in a sub, who isn't a regular employee of the district and doesn't get paid unless they are short staffed, and isn't certified, maybe has zero teaching experience, and has some college, but not necessarily in education.
But if a store or restaurant needs a cashier or waitress to come in … it has to be a sick employee, there's no way they could function with a substitute worker for a day or two.