It’s because what you think as good enough of a resolution for a camera, that point would be useless if your screen that displays the camera feed having a low resolution.

Then there is the aspect that if the lens covering the camera gets dirty then you won’t be able to see that well.

The camera won’t work for all conditions, think at night or in cloud that obscure visibility in flight.

The idea is good, however, as a primary means of determining a fault it won’t be that good for starters. Where it might be helpful is more to asses what might be happening if you are having something out the norms that isn’t covered by any training or emergency listed. Think the Qantas 32 that the engine failure led to multiple system failures. The camera would show exactly what systems are damaged but might give an idea, but also having a pilot walk down the aisle and look out the window under some circumstance would provide the same information, as long as conditions allow for viewing.

Currently some airlines have tail mounted and/or belly mounted cameras for passengers to view. If you look at that video feed, the camera placement and angles won’t make it good for assessing issues, but it’s perfectly fine for passengers to get a Birds Eye view of the flight.

To make the cameras useful the placement of the camera would be important and where it would need to be placed might not be practical.

Right now, the sensors and automatic monitoring provide better information than that of a camera.