Why can hard rock make it difficult to build a piled jetty?

In the context of civil engineering, building a piled jetty requires the seabed to be composed of material that allows for sufficient penetration by piles to ensure the structure's lateral stability.1

Hard rock is considered unsuitable for this construction method because it resists the penetration necessary to anchor the piles to the required depth. While some softer, friable rocks may be pierced using steel piles, solid hard rock generally prevents the effective driving of piles, making it difficult to secure the foundation for a jetty.1