It doesn't disappear when the date expires if nothing has been submitted because in situations where the assignment is being used as a genuine online assignment rather than as a reminder, it could still be outstanding with a late submission to come in. So where there is no submission, the assignment is - in most cases - still 'pending', even though it obviously is not in your use case.
One way around that - rather than the teacher going in manually to hide it or delete it - might be to use the conditional access features to set the dates for the assignment to be available - make it completely hidden after the due date using conditional access.