As in the case of fiscal rules, identifying the optimal speed limit under all circumstances is a highly complex task. Just as economic conditions can differ, so can driving conditions. But you cannot have speed limits that are constantly changing to suit all conditions. And you cannot have different speed limits for different drivers.
The technology simply does not exist or is too expensive to put in place. And the driver as well as the traffic authorities would, in any case, become confused. Speed limits need to be clear and enforceable. And you have to rely on the authorities to implement them in a way that makes good sense.