I think the problem is that Steam tries to prevent multiple people playing the game after one person has bought it.
For example, in the past you used to be able to buy a game and then "lend" it to your friend(s), and they could copy it and play it and then give it back to you.
Steam tries to enforce 1 user per purchase.
For example, with steam you can log into your account on any machine in the world, and play your games. So what's to stop you telling your friends your Steam login info and then all sharing 1 account? Well, if you try that, then your account will almost certainly get banned, as Valve log all the IP's that the account is used on. If you use it on too many IP's in too short a period of time, then they assume that more than one person is using the account, and they ban that account.
At the moment if someone has their Steam account information "hacked" then it is possible (in theory) to prove your ownership of the game and Valve can reclaim your account for you. This very much relies on the fact that the owner of the STEAM account is the owner of the game (CD key). It is almost impossible to PROVE you own a Steam account, but if you can prove you own a particular game (photocopy of box with CD key) then you prove you own the Steam Account. But if games (CD keys) can be transferred, then there is no link between a CD key and a Steam Account and it becomes much harder to prove ownership of either. There is then less that Valve could do in the case of an Account being hacked. At best you could just prove that you HAD (at some point) owned a particular game, but how could you prove you hadn't sold it on and were now trying to "claim" it back?
Also any (VAC) banned Steam accounts, result in any associated CD keys also being banned (for other games). Which could be messy if Steam Accounts and CD keys aren't PERMANENTLY tied together.
I can see that the system is frustrating when you want to sell on an old game. But it does seem that (at least in theory) the current system allows a certain amount of protection to genuine owners, that they maybe wouldn't get if games could be moved between accounts.