The problem is that once you purchase the game, you cannot return it, you cannot sell it, and you cannot give it away/transfer it to another party.

As well, despite the fact that the steam version has no packaging costs, no printing costs, no warehousing costs, no stocking, shipping, or handling costs, you are still paying the same for the game as everyone else who bought it in the store.

Finally, the Steam store does not answer to market concerns, and operates arbitrarily. For example: In most stores, once the demand for a game has worn off, the price comes down in order to move the remaining copies of a game to make room for new games. In the Steam store, costs remain the same until the vendor authorizes a price reduction based on arbitrary decisions (increase sales volume, allow for pricing difference between game and sequel, etc.).

The technology embedded in Steam would allow for the first issue to be resolved, should Valve care to pursue this. As well, a second Steam store, not operated by Valve, yet accessible on the Steam system would ensure that the last two issues are properly addressed.