A key aspect that you didn't mention is consistency. This is what explains the behavior of both sides. Customers generally know that Starbucks is not the best

A key aspect that you didn't mention is consistency. This is what explains the behavior of both sides. Customers generally know that Starbucks is not the best coffee you can get, but they don't have to be the best. Just like any chain, Starbucks primarily sells consistency. Consumers are risk-averse and going to a random shop to buy a coffee is a risk. It might be excellent but it might also be terrible. With Starbucks, you generally know what you're going to get and even though it's average, it's generally not terrible/un-drinkable so it's the safe choice.