Not all drivers can speak. Not all drivers speak in a way a voice control can reliably understand them. Not all drivers are in environments where voice commands can be easily understood, like loud music. If you are driving a car you likely have the ability to push a button.
And even when the driver can speak, use the language, be understood and the voice controls are reliable, sometimes they don't actually want to disturb the passengers sleeping in the car just to crack the sodding window open a bit!
