Like size and colour constancy, why does the brain not maintain focus constancy?
Interesting question. I concur with @Chris. Color constancy and size constancy are neural mechanisms and can therefore be imposed on visual input from the retina. In contrast, a blurry input cannot be neurally improved. As an analogy: Visual perceptions are processed and rendered by the brain and therefore represent "photoshopped" perceptions. In other words, perceptions are subjective representations manufactured by higher-level visual processing. However, focus is determined by optics. An out-of-focus picture, to push the analogy, cannot be fixed by fotoshop, nor the brain.