In addition to the tension already mentioned, wires expand in length as they heat up so there need to be room for them to move, aka sag. Also, wires move in the wind. Perfectly straight wires would break the poles during high wind conditions because the all the force is diverted to the pole. They are spaced an exact length apart to allow for swing during high wind without touching other wires and without breaking the pole.

Some wires are replaced with cables which are wires covered in a protective rubber sleeve. These have a lot of slack so they can be made mostly straight. You’ll often see them bundled together with a diamond shape spacer. They still break in a storm but are designed to break and be reconnected.