It's how a non-English-native speaker would build the adjective to Sisyphus. After all, that is his name.

My native tongue is German, and if I hadn't heard the word Sisyphean before, I would have tried to find it in the same manner. In German, tasks would be described as "Sisyphus labors", not via the adjective.

Thus, it would be reasonable for a German speaker to know the person's name more than the metaphorical word for "laborious + futile".