Why is there so much fascination with polls to begin with? I understand that there are betting markets, but it seems sort of silly. If you had a 100% accurate poll, for instance, then what would be the purpose of the actual election?
Polling is useful for candidates and gives them ideas on where to target outreach and spending.
For the rest of us, it gives us something to watch. With Presidential campaigns running for almost two years in advance of the actual voting weeks, there’s a huge gap between when the thing starts and when we see results. This way, people have something to fill the time. Even now that voting has started, we are still another 10 days until the voting is done and likely another 7 after that until we have a sufficient count to know who has been elected.
That’s a long time for a populace that’s worried, distracted, and interested, especially since so many of us live in states where—due to the mechanics of a broken election system—we can’t do much to influence the national outcome.
The only way that would work is if he made people more likely to lie about their voting intentions. Now, there may be something about Trump that makes polling methodologies less accurate (notably, many pollsters have started to take into account education, which turned out to be unexpectedly important last time round) but that points just to bad methodology, not inherent unpredictability.
To run a 100% accurate poll would require you to sample every voter, so it would literally be an election.
Actually an election is literally a poll in the sense of a sample, you are counting people at "polling stations" in order to gauge the public mood about who should be president.
When you see it that way, Nate Silver is predicting a sample of an unknown distribution, the "true" distribution of people's preferences.
The fact that you can only make one officially binding sample ("the voters") is a practicality, as is the fact that there's an electoral college that means votes have different values. The fact that turnout matters is another issue, a statistician might call it a sampling problem.