I mean... the 2016 result was well within the margin of error for most polls. Relative to any other indicator we have, they've been fairly reliable.
Not to mention the pause that was given by:
-Comey announcing an FBI investigation into Hillary weeks before election day.
-Clinton botching her campaign and ignoring key blue states when Trump rightfully pointed out "Where's Hillary while I'm here?", Minority and Independent voters, and undecided because she was assured she was the better choice than Trump and everyone saw that.
-Bernie or Bust. And I'm saying that as a Bernie voter. I still voted for Hillary.
-Citizens who believed voting to be a waste of time because of shenanigans on both sides (as Democratic states "lost" counties that were polling for Bernie).
This is the problem with America. Republicans come in, do some mess. We get all distraught. Democrat comes in, mends some things, and we start getting comfortable again. Republican comes in, says "We're doing great, but we can be doing better with you in charge!". Republican gets voted in, horrible things happen, Democrats get voted in to right it again. Rinse, repeat.
This is why I said I am starting to lose faith in Democracy. The process only works if you citizens who are concerned with the ACTUAL benefit of whatever you're trying to vote on. Not "Owning Libs no matter the cost"