Not all tai chi instructors ask to be addressed in a formal way.
Most tai chi classes are non-martial, so formality is unnecessary.
However, when the focus of the class is primarily martial, it is prudent to maintain a certain degree of discipline.
This ensures that students listen to the teacher and respond immediately to instructions.
If students get too excited and a situation becomes dangerous, the teacher needs to assert control quickly.
Calling your teacher "Dave" encourages a loose, casual connotation that is friendly and informal, but perhaps not so effective when seeking to manage a situation.