I think store workers often have a ‘shield’ that they put up to help make their job easier. To come from their perspective, as I’m a store worker myself, it would be exhausting to connect continuously to every customer, so there will be a handful who there is more conversation with but those are regular customers who come in often. Most of the time we’re focusing on the processes of our jobs.
I agree with those who say we tend to mirror those in front of us, normally a cheery “good morning”. will elicit a response, or a casual comment.