No, they are usually not finding a better job. They usually got too high, slept too long, had a family incident, or just "didn't feel like it". They usually go to another very similarly low-paid job. At this level of the employment market (Wal-Mart stockers, gas stations, big fast food chain kitchens), employers and employees both see each other as disposable and interchangeable. It is a two-way street.
Is it less expensive constantly hiring and training new people than it would be to pay enough to retain employees you already have?