I think the primary absurdity is in the contrast between Walter Mitty driving about 55 mph on the freeway in reality, while in his daydream he is navigating a hydroplane through a terrible storm.

The story is absurd in one sense because it stresses the essential isolation of the individual and lack of meaning in life. To escape this emptiness, Walter conjures up the these visions of heroism and excitement.

Why are all of Walter Mitty's daydreams so ridiculous in The Secret Life of Walter Mitty?

Each reader of James Thurber's story will use different adjectives to evaluate Walter Mitty's daydreams. They show his attitude toward life. Some dreams are very unlikely to happen to anyone, such as getting shot by a firing squad. Yet that dream communicates something essential about his state of mind: he wishes he were braver and feels that his life is threatened by the daily tedium and his wife's lack of understanding.

Mitty fantasizes about being a war hero as well, which seems like a noble goal. While we do not learn about any military service he performed, it is more likely that he did not serve. Dreaming of being a hero and helping others through a crisis also contrasts with his humdrum existence.