Because the cambered bar forces the lifter to maintain stricter form than a straight bar. I can get away with bowing my back, leaning into my toes, pressing with my quads, and all manner of bad things using a straight bar, but a cambered bar's pendulum action will force me stay balanced and recruit posterior leg and hip muscles. I don't envision the cambered bar replacing the straight bar, but it seems like a useful assistance exercise for lifters who struggle to stay balanced and recruit their posterior.
If you take a new lifter who can only low bar squat 95 pounds, their cambered bar squat will be virtually non-existent. Training their low bar squat for a period of time such that said lifter gets their low bar squat to 405 will probably approximate their cambered bar squat at around 300 pounds and they will have never touched the thing. Somehow, through magic, they developed the ability to maintain balance, posture, and form with a cambered bar despite never having used one. Conversely, training only the cambered bar up to 300 pounds will not give you a 405 low bar squat. These are not interchangeable adaptations.
You are falling into the same pitfall of thinking as the “functional fitness” folks: harder, more unstable, does not equate to better. Being stable with 405 low on your back is a much better way to train stability than a cambered bar at a lighter weight.
Certainly. What I'm wondering is whether the cambered bar forces the lifter to learn correct technique. We don't all have the benefit of a coach or a room full of full length mirrors to check form. Many of us are squatting in our garages with little, if any, idea of what's going on with our nether regions during the squat. It is in these scenarios where I envision a few sets with the cambered bar helpful as a mechanism to feel the correct motion of the squat. Otherwise, my concern is that I will simply learn and practice bad habits unawares.
Harder, more unstable, does not equate to better... unless we are comparing machines to barbells. Is it possible for the hypothetical 405 lifter you mention to practice dangerous, improper balance, posture, and form with a straight bar? Would squatting with a cambered bar make it more difficult for this lifter to practice improper form? If so, why would we not use the cambered bar occasionally to "get the feel" of a good, balanced squat?
Is correct technique not specific to the equipment? How does learning to squat with a cambered bar translate to the standard barbell? The cambered bar forces the lifter to learn correct technique with the cambered bar, since it is different than the standard bar. And everybody has a cell phone to video their sets. Mirrors are stupid, and people make do without a coach quite well, if necessary. If you want to squat with a trick bar, go ahead, but you have not made an argument for its use.