Why doesn't the black box also record video?
Why does the black box still only record the cockpit's audio


The Black Box

There isn't a single black box. There is normally a Flight Data Recorder (FDR) and a separate Cockpit Voice Recorder (CVR) in large civilian transport aircraft. These are not mandatory in military transports or smaller aircraft.
Under proposals made in the last decade or two there would probably be a separate video recorder that records the flight instrument panel, recording what information the displays are showing the pilots and the positions of switches. So far as I can find, there are no official proposals to record external views - which are usually likely to be much less useful in determining the cause of an incident if you have FDR and CVR data.



Opposition


  the National Transportation Safety Board in the United States has proposed the idea of featuring cockpit video recording in airliner cockpits several times since the year 2000. However, such proposals have largely been opposed, especially by pilot unions who view such a stipulation as an invasion of privacy. NTSB has never been met with support for such a requirement from the FAA, since first proposing it in the year 2000.


(my emphasis)

From ICAO is Not Calling for Cockpit Video Cameras in New Airplanes. Aviation Today. 2017.




  Pilots don’t see the disappearance of Flight 370 as a reason to embrace cameras. They cite two reasons for their opposition: Video surveillance will almost certainly be misinterpreted or get into the wrong hands, and it can adversely affect how they do their jobs. “What a camera can capture can be so easily misunderstood and misconstrued,” says Doug Moss, a former test pilot and accident investigator.


From Why Pilots Hate the Idea of Cameras Watching Them Fly. Wired. 2014.