How often do you think most people watch their speed gauges?
You and I might do so regularly, but you sure as hell cannot say that for sure about every other person on the road.
Furthermore, how obvious is the speed limit?
I can tell you with certainty that, outside of a few, mostly European, places, this may be unclear. North American traffic engineers happily design roads with speed limits anywhere between 40 and 80 km/h, with no changes to the cross-sectional geometry of the (st-) road.
Systemic speeding because of misguided road design is more common than you’d like to admit. And a few cameras probably only do so much to fix that.