Simply because users do not have input to make changes when the speed limits change. While traveling, I can't tell you how many times I missed the speed limit sign and had no clue what the speed limit was until I passed another sign, or decided to just go with the flow of traffic, actually have a speeding ticket as a result of the latter.
Even if you don't alert people that they are speeding (opt in/out feature ability), you could still have alerts like you do with car on the side of the road, or police, or debris in the road. Just a single alert that states... 'caution, reduced speed ahead', or 'speed limit increase ahead'.
With user input, this would be the most accurate listing of speed limits around, and could even allow for construction speed limits in certain cases.
In short.... put my vote in to add this feature!