>why there isn't a metric to distinguish between used cars that have been driven in an urban setting as opposed to highway cars where there is clearly a larger wear and tear Factor due to the fact that the miles placed on them are more strenuous miles on the car as opposed to a clean highway mile that affects more than just the engine output, we have it more gas mileage

Both - among others - are important and entirely separate metrics. Knowing the circumstances under which a car was driven is pointless without knowing the mileage. Knowing the mileage is pointless without any other data. A car's literal age is yet another important metric that has nothing to do with its mileage but is determined by looking at its build year.

There are dozens of other things that are important for determining and putting into context the condition of a used car. And any salesperson will provide you with the necessary details, not just the mileage.