Why do engineers put power lines so that they are slack in hot weather?

in the winter season materials such as rubber insulators and copper wire shrink and in the summer they expand just like wood. Therefore slack is needed on powerlines in relation to the weather so there is a little give to them. Engineers have nothing to do with the slack in the lines, it occurs naturally. In the sunlight objects expand a little due to the heat. This goes for roads, bridges, sidewalks, shingled roofs and hanging power lines. They sag more in the summer than in the winter. Engineers actually have to find ways so that they wont get slack. This expansion and contraction of any given material will actually shorten the life of it over time, and it will have to be replaced more often.