[runarberg]

There are a lot more cars, running on much greater speed, for a lot longer distance, with much greater weight, then humans walking on spiked shoes.

The masonry hammer runs on much greater speed, with much more force, a lot more often then the human steps on the peace of concrete being taken apart.

[fiblye]

Concrete sidewalks are pretty fragile. Unless they're freshly laid, most that I see have cracks and pits to some extent (probably mostly attributable to cooling/heating and water). Some people also seriously drop their feet down hard when they walk, and if they've got little bits of metal sticking out from the bottom of their shoes, I imagine that damage adds up and doubly so if it slams down an existing crack.