An exit survey works only to the extent that it can accurately mirror the voting universe. Pollsters will, when pressed, acknowledge the increasing number of citizens who decline to participate in surveys. Some voters do participate - and lie, for fun. Voting rules are getting more, not less, restrictive. Absentee ballots used to be available only to military overseas or people who could document that they'd be out of state on election day.
States increasingly let people vote by mail or in advance, as a convenience.
One of my daughters voted in Texas more than a week before the election - at her local supermarket. Exit pollsters were not posted at her supermarket checkout line. Perhaps I'm more jaundiced about exit polls than other journalists, or than I should be. If so, you can trace it to a mayoral election in Philadelphia in the 1980s when the newspaper where I worked partnered with a TV station to conduct an exit poll. The newspaper was using the data not to declare an outcome but to help us look for the evidence of where and how one would occur.