SVG basics - Why photographs aren't saved as vector images.

It comes from a massive difference in the way those two standards are and how the camera operates.

Pictures taken using a camera are saved as a raster graphic - which simply put is a rectangular grid of pixels - the camera takes the image in the front of it and 'translates' it to such grid. Pictures like that, depending on their resolution and color mode are normally very heavy in terms of their size, that's because each of those pixels has to be saved separately in a file. For that example standard HD resolution (1920×1080 pixels) consists of 2,073,600 pixels.

In case of vector graphics to make up an image geometrical primitives are being used such as points, lines, curves, and shapes or polygons. Files like that are much smaller in size as opposed to the raster graphics files. That comes from the fact that to draw an HD size square shape only four points would be necessary (each corner) they would be connected by a line between each of them and saved as a mathematical expression in XML format. That means that only few lines of text would be enough to describe such shape as opposed to over 2 millions pixels in case of raster graphics. The advantage of such format is that in can be infinitely scaled and it will preserve exactly the same quality in each size. Where raster graphics will lose its quality.

A vector graphic normally consists of a set of shapes stacked on top of each other (simply put). Cameras can only reflect a flat image and save it as the mentioned grid of pixels.

I recommend reading a bit more about those standards on wikipedia:

http://en.wikipedia.org/wiki/Raster_graphics

http://en.wikipedia.org/wiki/Vector_graphics

Hope that helps...