The big fat good economic sense.
If everyone had the three-phase 380v there would be even less copper.
Yes we have to balance three-phase power each 3 phases. Otherwise it starts messing.
Do not do at home. You would turn on at the same time your TV screen on a stage, a vacuum cleaner on another, and a halogen spotlight on another. And put out in same time.
There is that using a three-phase motor things work alone because it's intended for.