Well, I would think the obvious reason would be because you and a friend could simply gift the same game back and forth. Since your Steam already has the game files, the second you were gifted it, you'd be able to play it.
Now is this a Steam issue or a dev/pub issue? Do you think they want 10 people to be able to share the same purchase?
steam is all DRM, like its not hard for them to deauthorize it and remove your local game content? I doubt it.
but like juju also pointed out, I already owned a copy, and it just basically threw the other copy away.