Some good reasons:

-gotta hang them somehow, and in a very controlled way.

-thermal expansion, very important not to cause axial strain on a cable, which happens on tight bends.

You might think it would be enough to just have slack "somewhere" but I think you get to have many many micro adjustments when you have it across the entire length.

Why don't HV telephone lines do this?

I have no idea. Maybe because they can hang and droop more easily. I hope someone more knowledgeable gives a real answer.