A game costs 40$? Nahh, let's wait for a sale. Only 20$? Nahh, let's wait for 10$. Steam devalues games in the long run. And i dont know if thats what the majors really want.
The difference is that Steam sales don't happen as soon as a game is released. Steam sales are generally only offered on games that have been out for months and are well past their peak sales. Used sales devalue games immediately. When a game's first month profits determine whether or not a dev gets another contract or a game gets any patches or support, initial pricing is more important than late pricing.