And yet it works. They use the tools at hand to create a picture that turns out to be fairly accurate, within a margin of error that shrinks as sample size increases.. . . If you limit your universe of responses to just those people willing to pick up the phone, you have already introduced a ton of unaccounted-for bias. . . .
We can live with that. Sure, they could knock on doors or take clipboards to malls and gas stations or run ads on Facebook. but they'd still find people who don't want to talk with them. What they find is that phones work as well as anything.