ExploderWizard
Hero
The players are entrusting part of the creative and imaginative side of the game to whoever makes the setting. That doesn't mean they won't roleplay but it makes it likely that they won't get as involved in the design and personalisation of the game as a GMs who build their own campaigns based on their own and their players' imaginative input.
If the game's played at the end of everyone's working day, after a drive and in the middel of winter it's much easier to reach for the pre-packaged. Nothing wrong with that, especially if you wouldn't have a game otherwise.
At the same time though, part of what roleplaying games offer has been lost in asking others to help out.
I would say that this viewpoint isn't a universal truth. Some groups of players just won't care about the history of a podunk town they stroll into or any of its lovingly crafted NPC's. It won't matter if the DM picked it off a shelf of wrote every word personally. In fact, the DM who puts all that effort into worldbuilding for a group that doesn't care is wasting huge amounts of time.
Even within published settings there is room to create plenty of original material. If the DM is lucky enough to have a group that loves getting really into the setting then all that work can really enhance the game.