There are two kinds of images: rasters and vectors
Rasters are your typical image formats (bmp, png, JPEG) They aren't very good when scaled up or down.
Vectors, however, are pictures stores as a series of mathematical algorithms and so can be scaled up and down without any loss in image quality and are usually a smaller file size.
Why is the latter so uncommon?
-
The pure insanity of defining those equations perfectly. Think about it this way; if it were that easy, photo-realistic cgi would be child's play.
-
It would be nearly impossible to take an image produced by a camera and turn that into a vector without any loss of detail.
-
I imagine that not every image could be efficiently described with equations.
-
I like Vectors
-
Probably because of the limited technology in cameras. You can't zoom into every little detail with many cameras. Including the ones in phones.
-
Because most devices for taking and sharing pictures use rasters. Vector images are normally just used by professionals.