Because if they replaced a part of kerb they would have to do the whole pathway.
From the bucksfreepress.co.uk
I even managed to scrape its bottom on a kerb while reversing off a grass verge.
From the au.news.yahoo.com
To do this the kerb must be dropped to the road level ie at one of the entrances.
From the guardian.co.uk
It is also hard enough to have been used locally for pavements and kerb-stones.
From the en.wikipedia.org
They waited kerb-side while their friend parked his car and rushed back inside.
From the smh.com.au
Parking, especially parallel to the kerb, is something many people find difficult.
From the economist.com
Ford's Mark Winterbottom his the kerb as he attempts to overtake Jason Bright.
From the couriermail.com.au
I touched the kerb at 20mph and suddenly I was skidding along the road on my side.
From the guardian.co.uk
But Mrs Shaw fell between the kerb and the car and broke her femur, or thigh bone.
From the edp24.co.uk
More examples
Curb: an edge between a sidewalk and a roadway consisting of a line of curbstones (usually forming part of a gutter)
This article describes several characteristic architectural elements typical of European megalithic (Stone Age) structures.
A curb or kerb (see spelling differences) is the edge where a raised pavement/sidewalk/footpath, road median, or road shoulder meets an unraised street or other roadway. ...
The edge between the pavement and the roadway, consisting of a line of kerbstones; To damage vehicle wheels or tyres by running into or over a pavement kerb
(kerbing) A strip of kerb
Ring of retaining stones against mound or cairn base
Curb (noun). As in the step up at the side of a street.
A British term for a Curb -- a stone or cement ridge between the road and the sidewalk.
Usually made of long stones or concrete, this is the edge where a raised pavement meets a road. It acts as a barrier to stop motorists driving onto the pavement.