It is because your tongue does not perceive the chocolate as solid, but instead a liquid. In 1879 Rudolph Lindt invented a conching machine that smashed all the ingredients in his chocolate really small.
“a few microns (millionths of a meter, or 40 millionths of an inch) across, smaller than the human tongue and mouth can discern as individual grains. As far as perception goes, solids that are this fine feel as smooth as liquids”
Today I finished 
of Modernist Cuisine: The Art and Science of Cooking
“Unfortunately, blenders (whether upright or handheld) have an intrinsic limitation: they are generally unable to make particles smaller than 10-12 microns / 0.0004 – 0.0005 in, and even that is not easy to achieve. This dimension is just above the range of sizes that the human mouth can detect as a particle.”
What you need to make the smoothest purees or emulsions is a rotor-stator homogenizer which can smash up big time.