Actually, it's not. 3e had some very sketchy, but very key role-playing rules built into it. Part of that was the skill system (which 4e seems to be keeping but developing a little bit further). Part of that was the NPC system (which 4e is totally scrapping). Part of that was the monster system (which 4e is totally scrapping). Part of that was the system of spells and powers (which 4e is reimagining). Part of that was the system of treasures and rewards (which 4e is reimagining).
The Storyteller system has rules for morality.
To me, having rules for playing the role is the very essence of a roleplaying game.
Now, I know, especially with D&D, that part of the role I'm playing is a combat part. That's awesome and I'd embrace that fully for what it is.
But the reason I actually enjoy D&D is not because of the combat. It's because I get to be Achilles or King Arthur or Indiana Jones or Harry Potter or whatever. That's the essence, for me, of a heroic role-playing game.
D&D4e has shown that it's definatley going to let me play the role of a Striker or a Defender or a Controller or a Leader. Heck yeah it will!
But it hasn't shown me that it will let me play as these character archetypes.
Because these archetypes involve combat, oh yes, they definately do, and it's important to have that.
But they're not JUST combat, that's not the focus, that's not the reason.
There can and should be mechanics for these archetypes outside of combat.
The 3e skill system let me be an amazing diplomat or an intimidating warrior or a master crafter or a lore-filled sage or a master of exotic dance, and it let me do that mechanically. The 3e NPC system let me be uniquely powerful, above and beyond what the other people of the world could do, and it let me do that mechanically. The Storyteller system lets me fall from grace or struggle with sin, and it lets me do it mechanically.
It might just be that we haven't seen it all yet, but there's going to have to be a LOT more that they're not showing us to smooth this perception over.