First of all, there are no cameras on the planes because nobody asks them from the airlines. And the US Federal Aviation Administration and the European Aviation Safety Agency don’t see any point in that. But why?
In reality, the cameras are still installed on the plane, but only in certain places. For example, next to the door to the cockpit. This is necessary so that pilots can be sure that nothing threatens them before leaving the cockpit.
Some aircraft models can also be equipped with external cameras. So, in the Airbus A350 there is a camera in the lower part of the fuselage, as well as in the spine. This makes parking easier.
Ceilings on airplanes are quite low. Therefore, the cameras would have to be placed very close to the passengers and they would not be very comfortable with it.
And even if the ceilings on airplanes were high and there were cameras everywhere, the lining wouldn’t be any safer than that. Planes don’t have separate rooms for a security guard to sit and watch everything. Just because it’s expensive.