Idiocracy was not a prediction. Rather, it was a satire of the present, set in the future, much like Woody Allen's Sleeper. The future setting was merely an excuse to exaggerate the present. This is why Idiocracy appears predictive, because it's actually more related to the present than to the future. It's relatively easy, and indeed predictable, for an exaggeration of 2006 to become closer to reality in 2026. Almost every ridiculous element of the fictional future of the movie was based on the world we already know and knew.

The Simpsons "predicted" a Trump Presidency. Not because Matt Groening is the Kwisatz Haderach but simply because our society was already idiotic, so it was a natural extension of that idiocy. The US elected a B-movie (Bedtime for Bonzo) actor in 1980, so there was certainly precedent to the President.