It's not as simple as just adding a slider somewhere and calling it a day. If developer doesn't known the walk speed or it can be anything between 1m/s - 10m/s, then they need to take that account in designing combat, cut scenes, quests etc. If you speed everything up, what happens to sounds, physics etc.? If you set in stone that walk speed is 3m/s then you can safely design everything around that and not worry about anything which means more time to develop other features.
I agree, not everything can be made easily adjustable.
Still a more detailed difficulty setting shouldn't be too complicated, something like allowing to set more health for self,or a permanent buff that gives from start the maximum movement speed and similar.
I like how Diablo 3 has the difficulty slider: starting from really no challenge at all, like no way to die or lose, and gradually increasing it, by having 20 difficulty levels.
