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.
Options have a cost. You can't just make everything customisable by adding an UI element for it. All effort spent on adding options is cut from other features and content.
