There's the problem of needing escrow for selling the game in that scenario; if, say, they implement game resale as "You can ask Steam to cut your access and give you a unique code someone else can enter to gain access," there's the problem that if I buy that code from you, I have no knowledge of if it'll actually work when I get home (maybe you sold it twice?).

... and then Steam can offer payment via their in-store currency (with x% shaved off the top) to basically serve as an escrow intermediary. Win-win for everyone except the developers deprived of first-sales by the new resale market.