Why does an image get blurry beyond a limit of zooming?
Digital zooming is commonly done by using some form of interpolation, which commonly represents making up some form of smooth change between actual data samples.  Assuming the data was low pass filtered (or roughly near bandlimited) before sampling, the information about where any sharp (non-smooth) changes were located between samples has been lost.  Adding back any non-smoothly-changing details would be arbitrary (guessing from context, etc.), and thus possibly incorrect (non-corresponding to the optical field before sampling).  

Smooth changes or interpolations across a large area of pixels looks blurry to humans, since this effect does not occur in real life vision often unless one's eye or optical instrument is out-of-focus or not focused on the feature of interest.