Answer
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One doesn’t use higher voltages in household wiring, such as 600 V or 1200 V, because it is more dangerous. Accidental contact with a “live” wire would send 5-10 times as much current through someone’s body as compared to the typical 120V in US homes.
An additional consideration is that having a larger potential difference between conductors and grounded objects makes electrical breakdown of the air more likely, causing wires to be more likely to arc to other conductors, which increases the likelihood of short circuits, breakdown of electrical insulation, etc.
