Good point. My guess would be that it's probably because it's not a question that's asked frequently enough (and perhaps because SG doesn't want to flaunt loopholes in the system). :-)
Considering the sheer number of games in most accounts nowadays, most users would probably not even notice a few games they own appearing in their listed giveaways. Even if they do, I don't expect more than a few would bother to ask SG Support about it - they'd more likely just skip entering those.
The rare cases where such an entry wins a game is most likely handled by just requesting a new winner once detected. However, it would be interesting to know whether such a winner can get away with an extra key if the situation is not noticed by the giveaway creator or if there is some systemic check in place.

How could I not respond to someone who had the same conern I did some time ago?! :-)