Because you’re interacting with another human being who is there helping you. I’m guessing you’ve never worked retail/service before. People treat retail/service workers like shit all the time. To wear an AirPod while ordering is another distancing layer that dehumanizes the other person.
I always take out headphones when interacting with service workers, but I'd say at this point a solid 80% of them have their own airpods in even when taking orders or in a directly customer facing role. I think the social stigma has just changed over time.
