A camera lens, whether digital or traditional, refracts the light as it passes through. In some areas this refraction is so small is it not noticeable to the human eye. These are the areas we think of as "focused". If you blow the picture up large enough or zoom in close enough, you will quickly see that even in perfect focus, at a high enough resolution the image blurs.
This is a picture of a street in Koenji, Japan. For this article I have reduced it to 372x250, but that is the only change. As you can see, it is quite detailed with different fabrics in the store display and so on.
For example, here is a 372x250 area cut from the center of the picture at the original resolution.
Now watch what happens when I take this cropped portion, enlarge it to the same size as the original, and then take a 372x250 crop from the enlarged image.
It does not matter how expensive your camera and lens are, or how high the resolution of your digital image, at some point the result will be the same. If you enlarge the picture enough, it is reduced to a meaningless blur. Digital enhancement can help, but it cannot replace data that does not exist. The less well-focused the original image is, the greater the likelihood digital enhancement will have no impact at all.
Yes, I know, that's not how it works on your favorite crime drama. Such is the advantage of fiction over reality.
The natural refractive qualities of the lens and limits of the resolution of your recording media (whether digital or traditional) result in "circles of confusion" where the recording media simply cannot correctly record the light. These circles of confusion can be small and compact, creating a focused image, or loose and malformed, creating an unfocused image. It does not matter how good your photo enhancement software is, there is no substitute for a clearly focused original. At best, the software can increase contrast between each circle of confusion creating the illusion of sharper focus. The smaller and more compact these circles are, the better the end result when the software increases contrast.
A hundred years from now, things might be different. Resolutions will be high enough and substitution algorithms smart enough that the digital enhancement software will someday be able to create an image from a blurry mass, but those days are a long way off and even then, there will be inaccuracies.
Last but not least, here is the original image at the full resolution. (You'll have to click on this thumbnail to see it.)