Why do retail workers' hours get scheduled a week at a time?

Retail stores and places such as Starbucks, Domino’s Pizza, and Safeway all seem to have this practice: that workers don’t know their work hours more than a week in advance. Somebody, such as an assistant manager, prepares a schedule every week. It tends to vary from week to week, and until it comes out, people just don’t know when they’re going to be working.

What’s the reason or rationale for this? What’s the benefit? Why isn’t it better for everyone if regular steady employees, whether full or part time, have a basic schedule that they can just stick with? It’s impossible to make plans with them more than a week ahead because “I don’t know when I’m working next week” and “I haven’t seen my work schedule yet.” Why?

And doesn’t this constant uncertainty just make life harder for people who are probably working for minimum wage? Is there an upside?

Tags as I wrote them: workers, employment, work schedules, working hours, retail, jobs.