Telephone networks are fixed cost plants. The cost of the network is determined by the build-out at any given time, regardless of usage. The residential network is built according to certain assumptions regarding usage. People who use the Internet as dial-in users don't dial in all at once. They just don't.
People have complicated lives.
They eat, sleep, go to school, go to the movies, play with their children, and do other things which don't involve the Internet. We know that ISPs do not have one line per customer. They have far less than one line. According to responses from ISPs, most firms have between 10 and 20 customers per incoming line. That means simply that only 5 to 10 percent of their customers can be using the service at any given time.