And how would the cameras even work? Are the pilots supposed to switch between multiple camera feeds, or do we install dozens of screens? And then what, they see lots of black smoke on one camera, does that really tell them that much more than the ENG FIRE alert blaring in the background?
Maybe this could help during stable flight, but in this situation, when the pilots were likely already overloaded and probably had only a few seconds to escape this situation - if it was possible at all - I can't imagine it being helpful.
If we don't try to see how it goes, we won't know if it is a good idea or not.
It'd take lots of testing and engineering. But especially in cases where you have multiple warnings going off I imagine that a quick view at an exterior camera can often give you a clearer/faster indication of the situation