Because then if they need to to any kind of emergency replacement, or just replacement in general, they can hopefully only tear up one side of the road. And there's more under the street than just storm sewer. Not infinite space down there.How come all the manholes in many city streets are lined up right in the path of tire travel rather than centered in the lane where you would straddle them?
And why are they always a little lower than the road surface creating a bump?
Also, manholes sit lower because as they add asphalt lifts they don't always expose the entire ring and add more to lift it up. Or the contractor didn't get the correct elevation during construction, causing it to sit lower.