From importing lots of models into Blender, I think the reason is that other software treats things differently. For example, 3D Studio files are all triangulated faces - which is fine for game models, but useless when you want to model things and subsurf them - for that you want only (or mostly) quads.
. . . . and then there are free models that look like they just never got finished.
This, for example, is a free file that was once going to be a camaro:
[IMG]

[/IMG]