Over millions and millions of years of evolution, many mammals (including us) adapted our vision centers to quickly process large chunks of environmental information by developing a series of beneficial hard-wired processing networks in the visual cortex. These neural pathways allow us to do things like subconsciously notice small movement or even judge distances fairly accurately without parallax. Most of the time it works pretty well; I mean, it kept us as a species alive up to today. However, illusions like this "exploit" the "visual hacks" our brain uses (that allow us to process such large amounts of visual data so quickly) by creating objects that don't (or can't) exist in nature. Our brains are fairly picky about things like depth-of-field and textural shading and oftentimes their limits can be demonstrated by simply playing with a few colors and changing a few angles.