"but there's a whole world of people who, quite rightly, expect the person on the stage to entertain them"

Exactly, and any decent organiser will make sure they go back to their audience and get appraisals of each act who has been on.

Entertaining is the requirement and some people do not have that in them.

However, I know many people who sing and play so nicely, the entertaining is in their songs. Sometimes that is enough.