You can't keep a constant eye on both the road and the speedometer. Further, you might have missed the last speed limit change or remember it incorrectly. It's also possible for the speed sensor in your car to be faulty or out of calibration. This happens if you change the size of your wheels/tires significantly without reprogramming the ECU - and that setting isn't made available to the owner of the vehicle, at least not in most cars I'm aware of¹.

In fact, most cars lie to you about the speed - reporting a speed slightly faster than reality. It's a cover-your-ass measure for the car manufacturers because it's illegal to sell a car (in the US, at least) where the speedometer is inaccurate in the other direction, that is, reading slower than actual speed.

1. Some older vehicles, including pre-1996 GM trucks (and probably others from the same era) had the speedometer calibration controlled by a resistor array on a circuit board under the dash, those can be changed with a lot of effort and a soldering iron, or by swapping out the whole circuit board with a different one that matches your tire size + rear end gear ratio.