They do it because they can.
It costs money to remove cables. If the municipality doesn't require them to do so, why would they? Most people don't notice or complain, and there's the off chance they'll use it again if the next owner wants service.
I've gone so far as to cut them free of my home, neatly coil the cable, and wire it to the pole on the alley. Eventually they'll be at the pole for some other reason and cut it down. Technically the cable may be their property, but that doesn't mean it has to lay around on (or over) mine.