Ray,
I've seen curbs made out of asphalt.
You may well have. but it is still a "curb". So called because it "curbs" the pavement.
Definition
noun
1.
Also, British, kerb. a rim, especially of joined stones or concrete, along a street or roadway, forming an edge for a sidewalk.
2.
anything that restrains or controls; a restraint; check.
3.
an enclosing framework or border.
Besides we are talking about the curb on Elm Street, which wasn't made of asphalt, but concrete.