What is the fucking deal with my retail job acting like I'm the only person they have available to work my hours or fill in for others?

When I'm there, I'm the only person on the sales floor. When someone else calls out, they call on me to cover their shift. It's always me.

What the fuck is going on? I'm only working this job until I find a better one, but if they don't get their shit together, I'll just quit now.