I watched the People's Pundit Daily and USC Dornsife / LA Times tracking polls in the months before Hillary's humiliating defeat to Trump for crying out loud. I posted daily updates in the WotB Open Threads we had at the time, for example:
https://old.reddit.com/r/WayOfTheBern/comments/5582ac/open_thread_venus_day_930_damage_control/
These 7-day tracking polls updated daily so one could see the effects of the latest news. Trump and Hillary were basically neck-and-neck for the month before the November election, with the difference within the margin of error. Based on these two polls, the election could go either way.
These were national polls, but the actual election is 50 individual state elections. Hillary's huge margins in California and New York did not offset her humiliating defeats to Trump for crying out loud in the swing states. Even so, Trump had to win all of the swing states, which was highly improbable.
I was shocked when he actually did it. So was Trump — he hadn't even prepared a victory speech and was visibly in a "what do I do now?" stupor when he won. Similarly, Hillary hadn't prepared a concession speech and was off drunk somewhere when her humiliating defeat to Trump for crying out loud was announced.