They haven't, midterms don't count only the Presidency, and the majority of the forum has come of age politically during the Obama era. Basically every form of media they consume has said the GOP was dead forever for years. The most active posters have always had all the talking points at their fingertips, just look at Aaronology's fine work this election.
Just to take some examples, if you were:
12 in 2008, you're 20 now and first time voter.
15 in 2008, you're 23 now and you won last time if you're a Democrat.
18 in 2008, you're 26 now and never lost a Presidential election if you're a Democrat.
Hell, if you were 21 in 2008, you're 29 now, never lost a Presidential election and your first election was the Democrats grabbing the House and Senate for the first time in over a decade.
Even someone whose first time voting for President was 2004 is in their thirties.
And when you consider Bush "stole" both his elections, and the Democratic media wasn't exactly friendly to his regime, and the prior President was Bill Clinton. Plus everything has said for months that Clinton was a shoe-in and that Trump literally is a fascist and Hitler. Factor in how GAF in general operates on any topic. Existential dread is the most obvious reaction.
Which is why they're so eager to save the country by subscribing to The New York Times and The Washington Post. (Which they just spent six months bitching about for covering Hillary's scandals. And normalizing Trump.)