Because there are still a few million people without a home computer.
Internet is a basic human right in most developed countries, this is a non-issue.
The Internet, like many other things that are considered basic human rights in other countries, isn't a basic human right here. You can talk about using libraries and publicly-issues to fill the gaps, but, unfortunately, US politicians know how to game those things. They'll manipulate funding for public computers, change library hours and maybe even relocate libraries to gain a small electoral advantages.
And before you say they won't, that's exactly what happens today with polling places, distribution of voting machines and (in states that require photo ID to vote) locations and hours of DMVs (and any other places you can use to get a state-issued photo ID).
Available in: libraries, coffee places, public wifi, neighbours, friends, mobile phone, etc...