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I'm no electrician, but my understanding is that the phase of 120v has two opposite polarities, given the way AC power works. By pairing up two powerlines that are out of phase, you're getting 120v on the positive and 120v on the negative, which doubles the capacity along the parallel line. If you did this with a 110v half-line like the UK has, it increases the Amperage to an unsustainable level, while the US standard does not stress the line in the 240v configuration specifically because neither phase causes the other interference.

I normally don't say this, but don't trust my assessment. Google will be more accurate.