Because there will be more than double that number of cars on the road by that time, and so it will still result in a net increase. It's better than no cuts at all, but IMO,. they should be focusing on reducing emissions by at least 10% every single year to really stay ahead of the rate at which more cars are being added to roads... by the same time, at that rate, emissions would have been cut by about 80% off of what they are today.
That would make a difference. Reducing by 40% over 16 years is just political posturing, not any serious attempt at wanting to make the future any better than the present.