But in the final weeks the US election had close polling. Clinton was leading Trump by less than 5% in most polls and was actually behind in a few of them (Matt Drudge always managed to find those polls to link to).

So it's hard to understand why people might have those beliefs in a relatively close election. Easier to understand if the election was predicted to be a blowout.

But blowouts don't happen that often. The most recent true blowout I experienced was Reagan vs Mondale in 1984. An electoral blowout, only Minnesota and DC voted for Mondale. But the popular vote was closer, roughly 59%/41%.