That's not entirely true. Any load imbalance on the phases is resolved through the null, connected to earth at both ends of the line. You can do the same with t

That's not entirely true. Any load imbalance on the phases is resolved through the null, connected to earth at both ends of the line. You can do the same with three single phase lines, although you wound need a massive earthing system with low resistance. Or you could build a 2 wire (or 4, 6 ...) DC line with complementary polarity that would carry more power due to skin effects. If the negative and positive loads are balanced, the currents though the earthing wire cancel out.