Because, from Valve's perspective, there's no difference between the new games and the used games, except that Valve would get paid much less for the used ones and they would have to set up some system by which the consumer would pay both Valve and the seller. The amount of fraud that would attack such a system would be a nightmare, and they wouldn't get any more sales than they do now. You're asking Valve to make less money for more trouble, so what's their motive?
That's a good point about fraud, particularly with games not purchased directly from Valve--that would most definitely not be an option.
I'm only considering games purchased directly from Steam and resold to other Steam accounts through a payment system--that's an obstacle (CC companies get their cut, too) but not insurmountable.
In the end, Valve sells a game to a user = profit
User sells game to another user = profit - but not so much if the game sucks and the price drops, which is going to happen regardless.