Here's why:

* Abrasion is the process of rocks being worn down by friction. In this case, the friction is caused by the waves crashing against the rock, carrying sand and other sediment that act like sandpaper.

* This process gradually smooths the rough edges of the rock, making it more rounded.

Other types of weathering may also contribute to the overall shape of the rock, but abrasion is the primary factor in smoothing the edges.