They're paying shit wages and thus offering shit quality, because they're offering a service that literally is unscalable. The logistics of laser-guided-bomb-type delivery of groceries from store to fridge is asinine.
Yo, grocery store shopping doesn't suck bad enough to pay someone else to do both the shopping and delivering! I'll pick up the pre-picked-up/bought groceries. I don't need you to delivery them directly to my fridge; I think I can handle that part. Just bring them out to my car when I pull up.
Problem solved, wages go up, quality goes up, I get my groceries. Everybody wins.