(1) Masks are more effective at preventing transmission than reception.
(2) Different individuals have different levels of concern, which might lead one to choose a mask and another to choose not to wear one.
(3) Different individuals face different levels of risk. The person who is immunocompromised may wear a mask even when it would make no sense for other people.
(4) The customer may encounter 2-3 service employees in a day; the service employee may encounter hundreds of customers in the same time.
And that's without even getting into political issues (in the US, where mask-wearing has become politicized).
An acquaintance of mine worked in a casino that banned mask wearing by employees early on during the pandemic out of concern for worrying customers. Unfortunately several of his coworkers died of COVID before the lockdowns shut everything down.
Wouldn't the same apply to mask wearing? Yet we consistently see service employees wearing them and customers not.