You're correct, but according to some statistics, fewer than 40% of households have a "landline" phone.
This chart shows how landline phones are gradually losing relevance in the United States.
www.statista.com
Also, and most importantly, many who have a landline "home phone", may no longer have hard-wired "POTS" copper lines that will work in cases of power failure. When my parents signed up for a plan to get internet and home phone under 1 package, AT&T went to their place and installed a modem which now feeds their home phone service as well as their internet. This meant they no longer had "hard wired" phone service that would work during a power outage. When they called the phone company to complain, they were told that reconnecting their POTS service was not possible. When they explained they needed phone service in cases of power outages, the phone company provided them a UPS unit to supply power to their phone/internet modem for a few hours. See below from AT&T's website: