If you dig into the polls themselves you can see the sample distribution. (I.e. who is polled, Democrats, Republicans, or Independents) Also the polls assume people tell the truth when they are polled. We know from 2016 that even people who voted for Trump at the polls were too afraid to tell the exit pollers and thus the exit polls were totally wrong. The reason there will never be accurate polling is because of the Trump Effect of silent supporters, particularly those in urban areas/blue states who are too afraid to admit they support him.