Mostly asked why the tension is so high. On the other hand, I ask myself but why not chosen 1 mV.
There are high voltage lines with 1 mV and more, but not in Germany (e.g., in Kazakhstan).
The height of the voltage used is always a compromise. With increasing tension, the transmission losses decrease, but the construction becomes more and more expensive. The distances between the conductor ropes and also to the earth must become larger, the insulators are longer. This makes the masts higher and you need more space.
This is a problem in Germany (probably not in Kazakhstan).
That's why it leaves in Germany at 380 kV.