That way we could study plane crashes more closely, or terrorist hijackings, or resolve passenger disputes. Even buses in Ecuador have video cameras.(WARNING: shocking video)
Many airlines do in fact have inflight camera feeds for Cabin surveillance so I do not think the question is relevant to most major airlines anymore.
For a specific example, Emirates Airline employs the Cabin Video Monitoring Systems (CVMS) across its fleet of Airbus A380 aircraft. These Cabin cameras can be accessed by Cockpit crew through the System Display or the Cabin crew through the FAP (Flight Attendant Panel) and for any abnormal situation such as a hijacking/hostage situation on ground, the CVMS can be accessed on a laptop via the GSP (Ground Service Panel). Additionally, as mentioned above, they have landscape cameras installed for an external video feed but that is more of a feature for passengers.
The advantages of a CVMS in a hijacking/hostage situation such as positioning, identity, body language and weapons of the hijackers plus general operational advantages such as issues during boarding/offloading and other emergency situations such as an evacuation are great and not to be discounted.
Regarding cabin cameras, there's not much of a need for them. Busses typically don't have flight attendants. In-flight disputes are typically handled well and quickly by the cabin crew on an airliner. Also, theft or assault would have dozens to hundreds of eyewitnesses and no route of escape for the perpetrator, unlike on a bus (which is probably why such incidents are almost completely unheard of on airliners.) In the case of a terrorist hijacking, having video wouldn't really help much. It's usually pretty obvious when an airliner has been hijacked, but there's not a lot that people on the ground can do about it, aside from the possibility of shooting it down if it appears to be threatening a 9/11-style attack.
Regarding exterior cameras, these are actually starting to appear on some airliners now. Newer airliners like the 777-300ER and the A380, for instance, have them, though I don't think they actually record the video. Even if they did, it seems unlikely that they'd provide any useful information beyond what the FDR data and accident scene examination had already revealed (i.e. they'd basically just show that the plane ran into the ground, which would already be known by the time you found the video.)
Regarding cockpit video recording, there is an answer here that addresses it. There have been some suggestions for this to be added over the years, but pilots tend to be against it and the additional information it would provide would likely not be particularly useful in most instances, given that we already have a recording of all of the conversations up to the time limit of the recorder as well as the FDR data.
Note that cameras are already installed in some aircraft; but I believe this is an operator option and not standard equipment.
For example, from this incident report of a 737-800 PDF:
The captain stated that he could see on the cabin observation camera screen in the cockpit that a CCM was wearing personal breathing equipment (PBE). The captain switched off the IFE. This action also switched off the camera since the IFE and the camera are on the same electrical bus.
You will see them very soon. Actually, I have already made the software for that :) and we will be delivering to few of the Airlines very soon as I work on those systems where we will have the DVR (Digital Video Recorder) and and a live streaming panel. This recording is only manually controlled just by push of a button by pilot or crew. You will see them in B787s. There are few in A350 also for few airlines.