Why are safety demonstrations done while taxiing? In airliners, safety demonstrations are performed when the aircraft is leaving the gate. I understand it cannot be done latter as an aircraft incident including an evacuation may occur at take off roll. But I think it could be done earlier (e.g. in waiting area before boarding). This way, if any passenger has any question (e.g. this one), this passenger has plenty of time to ask flight attendants before takeoff. This time may also be used to explain why safety demonstration include all those items (life vest inflated outside the airplane is not the only item that could raise questions). Moreover it could be a good use of waiting time before boarding.

EDIT: As highlighted by one answer, some airliners use video to do this demonstration. In this case, the generic items (i.e. not aircraft specific) of the video could be played on screen in waiting area in addition to the actual briefing.
Many passengers don't reach the gate until seconds before the gate closes for a variety of reasons. They won't get the pre-boarding briefing. 

Taxiing is the only time before takeoff where the crew knows for sure that all passengers that will be on the plane are there.