In addition to all the other reasons:

• Every camera-to-screen system has lag. Lag in a dogfight can be fatal.
• There will be some distortion between the camera and the screen, no matter how well designed. Distortion can be fatal too.
• Cameras and screen cannot reproduce depth perception (yes there are ways to do it but they are a LOT more complicated than cameras and screens). Lack of depth perception can be fatal in lots of situations.
• It is possible to get positional cues (especially depth cures) for a visible object by moving your head, even the small amount of movement that a fighter pilot's head can make. A camera screen system won't do that.

A screen system can be useful if the alternative is not being able to see in some direction at all, but you wouldn't replace direct vision with it.

In addition to all the other reasons:

• Every camera-to-screen system has lag. Lag in a dogfight can be fatal.
• There will be some distortion between the camera and the screen, no matter how well designed. Distortion can be fatal too.
• Cameras and screen cannot reproduce depth perception (yes there are ways to do it but they are a LOT more complicated than cameras and screens). Lack of depth perception can be fatal in lots of situations.
• It is possible to get positional cues (especially depth cues) for a visible object by moving your head, even the small amount of movement that a fighter pilot's head can make. A camera screen system won't do that.

A screen system can be useful if the alternative is not being able to see in some direction at all, but you wouldn't replace direct vision with it.